INFECTION

CONCEPT

Humans may hold dominance over most other life-forms on Earth, but a few varieties of organism have long held mastery over us. Ironically, these life-forms, including bacteria and viruses, are so small that they cannot be seen, and this, in fact, has contributed to their disproportionate influence in human history. For thousands of years, people attributed infection to spiritual causes or, at the very least, to imbalances of "humors," or fluids, in the human body. Today germ theory and antisepsis—the ideas that microbes cause infection and that a clean body and environment can prevent infections—are ingrained so deeply that we almost take them for granted. Yet these concepts are very recent in origin, and for a much longer span of human history people quite literally wallowed in filth—with predictable consequences.

HOW IT WORKS

What Is Infection?

The term infection refers to a state in which parasitic organisms attach themselves to the body, or to the inside of the body, of another organism, causing contamination and disease in the host organism. Parasite refers generally to any organism that lives at the expense of another organism, on which it depends for support. Numerous parasites and the diseases they cause are discussed in the essay Parasites and Parasitology; in the present context, we are concerned primarily with infections that relate to bacteria and viruses.

Almost all infections contracted by humans are passed along by other humans or animals.

Infections fall into two general categories: exogenous, or those that originate outside the body, and endogenous, which occur when the body's resistance is lowered. Examples of exogenous infection include catching a cold by drinking after someone else from the same glass; coming down with salmonella after ingesting under-cooked eggs, meat, or poultry; getting rabies from a dog bite; or contracting syphilis, AIDS (acquired immunodeficiency syndrome), or some other sexually transmitted disease from an infected partner.

Any number of factors—lack of sleep, prolonged exposure to extreme cold or moisture, and so on—can lower the body's resistance, opening the way for an endogenous infection. Malnutrition, illness, and trauma also can be factors in endogenous infection. Substance abuse, whether it be the use of tobacco in its many forms, excessive drinking, or drug use, lowers the body's resistance. Furthermore, all of these behaviors tend to be coupled with poor eating habits, which invite infection by denying the body the nutrients it needs.

Some Terms

A whole array of terminology attends the study of infection and infectious diseases, a subject that is touched upon in the present context but explored at length in its own essay as well. Among these terms are the names for the different branches of study relating to infection, its agents, and the resulting diseases. Although germ theory is a term (defined later) that is used widely in the context of infection, germ itself—a common word in everyday life—is not used as much as microorganism or pathogen. The latter word refers to disease-carrying parasites, which are usually microorganisms. Two of the principal types of pathogen, bacteria and viruses, are discussed later in this essay.

Words relating to the effects of infectious agents include epidemic, an adjective meaning "affecting or potentially affecting a large proportion of a population"; as a noun, the word refers to an epidemic disease. Pandemic also doubles as an adjective, meaning "affecting an extremely high proportion of a population over a wide geographic area," and a noun, referring to a disease of pandemic proportions. Areas of study relating to pathogens, their effects, and the prevention of those effects include the following.

• Bacteriology: An area of the biological sciences concerned with bacteria, including their importance in medicine, industry, and agriculture)
• Epidemiology: An area of the medical sciences devoted to the study of disease, including its incidence, distribution, and control within a population)
• Etiology: A branch of medical study concerned with the causes and origins of disease. Also, a general term referring to all the causes of a particular disease or condition)
• Immunology: The study of the immune system, immunity, and immune responses)
• Pathology: The study of the essential nature of diseases)
• Virology: The study of viruses

In addition, there are several terms relating to the prevention of infection.

• Antibiotic: A substance produced by, or derived from, a microorganism, which in diluted form is capable of killing or at least inhibiting the action of another microorganism. Antibiotics typically are not effective against viruses.
• Antisepsis: The practice of inhibiting the growth and multiplication of microorganisms)
• Germ theory: A theory in medicine, widely accepted today, that infections, contagious diseases, and other conditions are caused by the actions of microorganisms)
• Immunity: A condition of being able to resist a particular disease, particularly through means that prevent the growth and development of pathogens or counteract their effects
• Inoculation: The prevention of a disease by the introduction to the body, in small quantities, of the virus or other microorganism that causes the disease
• Vaccine: A preparation containing microorganisms, usually either weakened or dead, which are administered as a means of increasing immunity to the disease caused by those microorganisms

Some of these words appear in this essay and others in related essays on infectious diseases and immunity.

Bacteria

Five major groups of microorganisms are responsible for the majority of infections. They include protozoa and helminths, or worms—both of which are considered in Parasites and Parasitology—as well as bacteria and viruses. Bacteria and viruses often are discussed, along with fungi (the fifth major group), in the context of infection and infectious diseases. In the present context, however, we limit our inquiry to viruses and bacteria.

Bacteria are very small organisms, typically consisting of one cell. They are prokaryotes, a term referring to a type of cell that has no nucleus. In eukaryotic cells, such as those of plants and animals, the nucleus controls the cell's functions and contains its genes. Genes carry deoxyribonucleic acid (DNA), which determines the characteristics that are passed on from one generation to the next. The genetic material of bacteria is contained instead within a single, circular chain of DNA.

Members of kingdom Monera, which also includes blue-green algae (see Taxonomy), bacteria generally are classified into three groups based on their shape: spherical (coccus), rodlike (bacillus), or spiralor corkscrew-shaped (spirochete). Some bacteria also have a shape like that of a comma and are known as vibrio. Spirochetes, which are linked to such diseases as syphilis, sometimes are considered a separate type of creature; hence, Monera occasionally is defined as consisting of blue-green algae, bacteria, and spirochetes.

The cytoplasm (material in the cell interior) of all bacteria is enclosed within a cell membrane that itself is surrounded by a rigid cell wall. Bacteria produce a thick, jellylike material on the surface of the cell wall, and when that material forms a distinct outer layer, it is known as a capsule. Many rod, spiral, and comma-shaped bacteria have whiplike limbs, known as flagella, attached to the outside of their cells. They use these flagella for movement by waving them back and forth. Other bacteria move simply by wiggling the whole cell back and forth, whereas still others are unable to move at all.

Bacteria most commonly reproduce by fission, the process by which a single cell divides to produce two new cells. The process of fission may take anywhere from 15 minutes to 16 hours, depending on the type of bacterium. Several factors influence the rate at which bacterial growth occurs, the most important being moisture, temperature, and pH, or the relative acidity or alkalinity of the substance in which they are placed.

Bacterial preferences in all of these areas vary: for example, there are bacteria that live in hydrothermal vents, or cracks in the ocean floor, where the temperature is about 660°F (350°C), and some species survive at a pH more severe than that of battery acid. Most bacteria, however, favor temperatures close to that of the human body—98.6°F (37°C)—and pH levels only slightly more or less acidic than water. Since they are composed primarily of water, they thrive in a moist environment.

Viruses

One of the interesting things about bacteria is their simplicity, coupled with the extraordinary complexity of their interactions with other organisms. As simple as bacteria are, however, viruses are vastly more simple. Furthermore, the diseases they can cause in other organisms are at least as complex as those of bacteria, and usually much more difficult to defeat. Whereas there are "good" bacteria, as we shall see, scientists have yet to discover a virus whose impact on the world of living things is beneficial. There is something downright creepy about viruses, which are not exactly classifiable as living things; in fact, a virus is really nothing more than a core of either DNA or RNA (ribonucleic acid), surrounded by a shell of protein.

Two facts separate viruses from the world of the truly living. First, unlike all living things (even bacteria), viruses are not composed of even a single cell, and, second, a virus has no life if it cannot infect a host cell. When we say "no life" in this context, we truly mean no life. Although parasites, including bacteria and those species discussed in Parasites and Parasitology, depend on other organisms to serve as hosts, they can live when they are between hosts. They are rather like a person between jobs: without other means of support, the person eventually will go broke or starve, but typically such a person can hang on for a few months until he or she finds a new job. A virus without a host, on the other hand, is simply not alive—not dead, like a formerly living thing, but more like a machine that has been switched off.

Once a virus enters the body of a host, it switches on, and the result is truly terrifying. In order to produce new copies of itself, a virus must use the host cell's reproductive "machinery"—that is, the DNA. The newly made viruses then leave the host cell, sometimes killing it in the process, and proceed to infect other cells within the organism. As for the organisms that viruses target, their potential victims include the whole world of living things: plants, animals, and bacteria. Viruses that affect bacteria are called bacteriophages, or simply phages. Phages are of special importance, because they have been studied much more thoroughly than most viruses; in fact, much of what virologists now know about viruses is based on the study of phages.

REAL-LIFE APPLICATIONS

Bacteria and Humans

Not all bacteria are harmful; in fact, some even are involved in the production of foods consumed by humans. For example, bacteria that cause milk to become sour are used in making cottage cheese, buttermilk, and yogurt. Vinegar and sauerkraut also are produced by the action of bacteria on ethyl alcohol and cabbage, respectively. Other bacteria, most notably Escherichia coli (E. coli ) in the human intestines, make it possible for animals to digest foods and even form vitamins in the course of their work. (See Digestion for more on these subjects.) Others function as decomposers (see Food Webs), aiding in the chemical breakdown of organic materials, while still others help keep the world a cleaner place by consuming waste materials, such as feces.

Despite its helpful role in the body, certain strains of E. coli are dangerous pathogens that can cause diarrhea, bloody stools, and severe abdominal cramping and pain. The affliction is rarely fatal, though in late 1992 and 1993 four people died during the course of an E. coli outbreak in Washington, Idaho, California, and Nevada. More often the outcome is severe illness that may bring on other conditions; for example, two teenagers among a group of 11 who became sick while attending a Texas cheerleading camp had to receive emergency appendectomies. The pathogen is usually transmitted through under-cooked foods, and sometimes through other means; for example, a small outbreak in the Atlanta area in the late 1990s occurred in a recreational water park.

BACTERIAL INFECTIONS.

Many bacteria attack the skin, eyes, ears, and various systems in the body, including the nervous, cardiovascular, respiratory, digestive, and genitourinary (i.e., reproductive and urinary) systems. The skin is the body's first line of defense against infection by bacteria and other microorganisms, although it supports enormous numbers of bacteria itself. Bacteria play a major role in a skin condition that is the bane of many a young man's (and, less frequently, a young woman's) existence: acne. Pimples or "zits," known scientifically as Acne vulgaris, constitute one of about 50 varieties of acne, or skin inflammation, which are caused by a combination of heredity, hormones, and bacteria—particularly a species known as Propionibacterium acnes. When a hair follicle becomes plugged by sebum, a fatty substance secreted by the sebaceous, or oil, glands, this forms what we know as a blackhead; a pimple, on the other hand, results when a bacterial infection, brought about by P. acnes, inflames the blackhead and turns it red. For this reason, antibiotics may sometimes cure acne or at least alleviate the worst symptoms.

Acne may seem like a life-and-death issue to a teenager, but it goes away eventually. On the other hand, toxic shock syndrome (TSS), caused by other bacteria at the surface of the skin—species of Staphylococcus and Streptococcus —can be extremely dangerous. The early stages of TSS are characterized by flulike symptoms, such as sudden fever, fatigue, diarrhea, and dizziness, but in a matter of a few hours or days the blood pressure drops dangerously, and a sunburn-like rash forms on the body. Circulatory problems arise as a result of low blood pressure, and some extremities, such as the fingers and toes, are deprived of blood as the body tries to shunt blood to vital organs. If the syndrome is severe enough, gangrene may develop in the fingers and toes.

In 1980, several women in the United States died from TSS, and several others were diagnosed with the condition. As researchers discovered, all of them had been menstruating and using high-absorbency tampons. It appears that such tampons provide an environment in which TSS-causing bacteria can grow, and this led to recommendations that women use lower-absorbency tampons if possible, and change them every two to four hours. Since these guidelines were instituted, the incidence of toxic shock has dropped significantly, to between 1 and 17 cases per 100,000 menstruating women.

Many bacteria produce toxins, poisonous substances that have effects in specific areas of the body. An example is Clostridium tetani, responsible for the disease known as tetanus, in which one's muscles become paralyzed. A related bacterium, C. botulinum, releases a toxin that causes the most severe form of food poisoning, botulism. Salmonella poisoning comes from another genus, Salmonella, which includes S. typhi, the cause of typhoid fever.

Viral Infections

With viruses, as we have noted, there is no need even to discuss "good" kinds, because there is no such thing—all viruses are harmful, and most are killers. The particular strains of virus that attack animals have introduced the world to a variety of ailments, ranging from the common cold to AIDS and some types of cancer. Other diseases related to viral infections are hepatitis, chicken pox, smallpox, polio, measles, and rabies.

One reason why physicians and scientists have never found a cure for the common cold is that it can be caused by any one of about 200 viruses, including rhinoviruses, adenoviruses, influenza viruses, parainfluenza viruses, syncytial viruses, echoviruses, and coxsackie viruses. Each has its own characteristics, its favored method of transmission, and its own developmental period. These viruses can be transmitted from one person to another by sneezing on the person, shaking hands, or handling an object previously touched by the infected person. Surprisingly, some more direct forms of contact with an infected person, as in kissing, seldom spread viruses.

A group of viruses called the orthomyxoviruses transmit influenza, an illness usually characterized by fever, muscle aches, fatigue, and upper respiratory obstruction and inflammation. The most common complication of influenza is pneumonia, a disease of the lungs that may be viral or bacterial. The viral form of pneumonia that goes hand in hand with influenza can be very severe, with a high mortality (death) rate; by contrast, bacterial pneumonia, which typically appears five to ten days after the onset of flu, can be treated with antibiotics.

THE EVER ELUSIVE VIRUS.

Viruses are tricky. Because their generations are very short and their structures extremely simple, they are constantly mutating (altering their DNA and hence their heritable traits) and thus becoming less susceptible to vaccines. This is the reason why flu vaccine has to be prepared a new each year to target the current strains, and even then the vaccine is far less than universally effective. On the other hand, vaccination has a high rate of success for strains of virus that undergo little mutation—for example, the smallpox virus.

One particularly elusive type of virus is known as a retrovirus, which reverses the normal process by which living organisms produce proteins. Ordinarily, DNA in the cell's nucleus carries directions for the production of new protein. Coded messages in the DNA molecules are copied into RNA molecules, which direct the manufacture of new protein. In retroviruses, that process is reversed, with viral RNA used to make new viral DNA, which then is incorporated into host cell DNA, where it is used to direct the manufacture of new viral protein. Among the diseases caused by retroviruses is AIDS, discussed in Infectious Diseases and The Immune System.

Fighting the Invisible War

Every day of our lives, we are at war with microorganisms, both individually and as a species. It is a war that has lasted for several million years, with billions of lives in the balance, yet it is an invisible war. Up until a few centuries ago, in fact, we had no idea what we were fighting. Before the advent of germ theory, the most scientific theories of disease blamed them either on an imbalance of "humors" (blood, phlegm, yellow bile, and green bile), or on inhaling bad air. These were the most advanced ideas, the ones held by men of learning; most of the populace, by contrast, believed that disease was caused by evil spirits, cast upon individuals or populations by an angry God as punishment for disobedience.

Personal hygiene and public health were completely foreign concepts: not only did people bathe infrequently, but they also thought nothing of throwing trash—including rotting food and even human excrement—into the city streets. This image of trash in the streets may call to mind a city of medieval Western Europe, a place and time widely known for its filth, squalor, and ignorance. Yet such an image also describes Athens during the fifth century b.c., when human imagination, wisdom, and appreciation for beauty reached perhaps their highest points in all of history. In the Athens of Socrates, Herodotus, Hippocrates, and Sophocles, the streets were piled with trash and crawling with vermin. In fact, this lack of concern for cleanliness contributed directly to the end of the Greek golden age, sometimes known as the Age of Pericles, after Athens's great leader (495-425 b.c.)—who died in a great plague that swept the germ-ridden city.

BACTERIOLOGY AND ANTI-SEPSIS.

The first inkling of any etiology other than that of imbalanced humors and demons was the work of the Italian physician Girolamo Fracastoro (ca. 1483-1553), who put forth the theory that disease is caused by particles so small they are almost imperceptible. The invention of the microscope in 1590 made it possible to glimpse those particles, which Holland's Anton van Leeuwenhoek (1632-1723)—the first human being to observe bacteria and other microorganisms—dubbed animalcules, or "tiny animals." The German scholar Athanasius Kircher (1601-1680) also observed "tiny worms" in the blood and pus of plague victims and theorized that they were the source of the infection. This was the first theory that dealt with microbial agents as infectious organisms.

In 1848 Ignaz P. Semmelweis (1818-1865), a Hungarian physician working in German hospitals, came up with a novel idea: after examining the bodies of women who had died of puerperal (childbed) fever, he suggested that doctors should wash their hands in a solution of chlorinated lime water before touching a pregnant patient. Semmelweis's idea resulted in a drastic reduction of puerperal fever cases, but his colleagues denounced his outlandish notion as a useless and foolish waste of time. Six years later, in 1854, modern epidemiology was born when the English physician John Snow (1813-1858) determined that the source of a cholera epidemic in London could be traced to the contaminated water of the Broad Street pump. After he ordered the pump closed, the epidemic ebbed—and still many physicians refused to believe that invisible organisms could spread disease.

GERM THEORY.

A major turning point came just three years later, in 1857, when the great French chemist and microbiologist Louis Pasteur (1822-1895) discovered that heating beer and wine to a certain temperature killed bacteria that caused these liquids to spoil or turn into vinegar. Thus was born the process of pasteurization, still used today to purify such foods as milk, because, as Pasteur observed, "There are similarities between the diseases of animals or man and the diseases of beer and wine." Pasteur also dealt the final blow to spontaneous generation, a centuries-old belief that living organisms could originate from nonliving matter. As he showed in 1861, microorganisms present in the air can contaminate solutions that seem sterile.

Then, in 1876, the German physician Robert Koch (1843-1910) proved what Kircher had postulated two centuries earlier: that bacteria can cause diseases. Koch showed that the bacterium Bacillus anthracis was the source of anthrax in cattle and sheep and generalized the methodology he had used in that situation to form a specific set of guidelines for determining the cause of infectious diseases. Known as Koch's postulates, these guidelines define a truly infectious agent as one that can be isolated from an infected animal, cultured in a laboratory setting, introduced into a healthy animal to produce the same infection as in the first animal, and isolated again from the second animal. These ideas formed the basis of research into bacterial diseases and are still dominant in the sciences devoted to the study of disease.

Koch's postulates helped usher in what has been called the golden era of medical bacteriology. Between 1879 and 1889 German microbiologists isolated the organisms that cause cholera, typhoid fever, diphtheria, pneumonia, tetanus, meningitis, and gonorrhea as well the Staphylococcus and Streptococcus organisms. Even as Koch's work was influencing the development of the germ theory, the influence of the English physician Joseph Lister (1827-1912) was being felt in operating rooms. Building on the work of both Semmelweis and Pasteur, Lister—for whom the well-known antiseptic mouthwash Listerine was named—began soaking surgical dressings in carbolic acid, or phenol, to prevent postoperative infection.

ANTIBIOTICS.

Whereas antisepsis was the great battleground of the invisible war during the nineteenth century, in the twentieth century the most important struggle concerned the development of antibiotics. The first effective medications to fight bacterial infection in humans were sulfa drugs, developed in the 1930s. They work by blocking the growth and multiplication of bacteria and were initially effective against a broad range of bacteria, but many strains of bacteria have evolved resistance to them. Today, sulfa drugs are used most commonly in the treatment of urinary tract infections and for preventing infection of burn wounds.

The importance of sulfa drugs was eclipsed by that of penicillin, first discovered in 1928 by the British bacteriologist Alexander Fleming (1881-1955). Working in his laboratory, Fleming noticed that a mold that had fallen accidentally into a bacterial culture killed the bacteria. Having identified the mold as the fungus Penicillium notatum, Fleming made a juice with it that he called penicillin. He administered it to laboratory mice and discovered that it killed bacteria in the mice without harming healthy body cells.

It would be more than a decade before the development of a form of penicillin that could be synthesized easily. This drug arrived on the scene in 1941—just in time for the years of heaviest fighting in World War II—and after the war pharmaceutical companies began to manufacture numerous varieties of antibiotic. By the last decade of the twentieth century, however, a new problem emerged: bacteria were becoming resistant to antibiotics. This has been the case with medications used to treat conditions ranging from children's ear infections to tuberculosis.

An example is amoxicillin, a penicillin derivative developed in the late twentieth century. Many pediatricians found it a better treatment than penicillin for ear infections, because it did not tend to cause allergic reactions sometimes associated with the other antibiotic. However, by the late 1990s evidence surfaced indicating that certain types of bacteria had developed a protein that rendered amoxicillin ineffective against ear infections. Critics of amoxicillin (or of antibiotic treatments in general) maintained that widespread prescription of the antibiotic actually helped create that situation, because the bacteria developed the protein mutation defensively. Because of these and similar concerns associated with antibiotics, doctors have begun taking measures toward controlling the spread of antibiotic-resistant diseases, for instance by prescribing antibiotics only when absolutely necessary. Research into newer types and combinations of drugs is ongoing, as is research regarding the development of vaccines to prevent bacterial infections.

WHERE TO LEARN MORE

Biddle, Wayne. A Field Guide to Germs. New York: Henry Holt, 1995.

The Big Picture Book of Viruses. Tulane University (Web site). <http://www.tulane.edu/~dmsander/Big_Virology/BVHomePage.html>.

Cells Alive! (Web site). <http://www.cellsalive.com/>.

Centers for Disease Control and Prevention (Web site). <http://www.cdc.gov/>.

Infection Index. Spencer S. Eccles Health Sciences Library, University of Utah (Web site). <http://medlib.med.utah.edu/WebPath/INFEHTML/INFECIDX.html>.

"Oral Health Topic: Infection Control." American Dental Association (Web site). <http://www.ada.org/public/topics/infection.html>.

The Race Against Lethal Microbes: Learning to Outwit the Shifty Bacteria, Viruses, and Parasites That Cause Infectious Diseases. Chevy Chase, MD: Howard Hughes Medical Institute, 1996.

Virtual Museum of Bacteria. Bacteria Information from the Foundation for Bacteriology (Web site). <http://www.bacteriamuseum.org/>.

Weinberg, Winkler G. No Germs Allowed!: How to Avoid Infectious Diseases at Home and on the Road. New Brunswick, NJ: Rutgers University Press, 1996.

KEY TERMS

ANTIBIOTIC:

A substance produced by or derived from a microorganism, which in diluted form is capable of killing or at least inhibiting the action of another microor ganism. Antibiotics are not usually effective against viruses.

ANTISEPSIS:

The practice of inhibiting the growth and multiplication of microorganisms, generally by ensuring the cleanliness of the environment.

BACTERIOLOGY:

An area of the bio logical sciences concerned with bacteria, including their importance in medicine, industry, and agriculture.

DNA:

Deoxyribonucleic acid, a molecule in all cells, and many viruses, containing genetic codes for inheritance.

ENDOGENOUS:

A term for an infection that occurs when the body's resistance is lowered. Compare with exogenous.

EPIDEMIC:

Affecting or potentially affecting a large proportion of a popula tion (adj. ) or an epidemic disease (n. )

EPIDEMIOLOGY:

An area of the medical sciences devoted to the study of disease, including its incidence, distribution, and control within a population.

ETIOLOGY:

A branch of medical study concerned with the causes and origins of disease; also, a general term referring to all the causes of a particular disease or condition.

EXOGENOUS:

A term for an infection that originates outside the body. Compare with endogenous.

GENE:

A unit of information about a particular heritable (capable of being inherited) trait that is passed from parent to offspring, stored in DNA molecules called chromosomes.

GERM THEORY:

A theory in medicine, widely accepted today, that infections, contagious diseases, and other conditions are caused by the actions of microorganisms.

IMMUNITY:

The condition of being able to resist a particular disease, particularly through means that prevent the growth and development of pathogens or counteract their effects.

IMMUNOLOGY:

The study of the immune system, immunity, and immune responses.

INFECTION:

A state or condition in which parasitic organisms attach them selves to the body or to the inside of the body of another organism, producing contamination and disease in the host.

INOCULATION:

The prevention of a disease by the introduction to the body, in small quantities, of the virus or other microorganism that causes the disease.

MUTATION:

Alteration in the physical structure of an organism's DNA, resulting in a genetic change that can be inherited.

PANDEMIC:

Affecting an extremely high proportion of a population over a wide geographic area (adj. ) or a disease of pandemic proportions (n. )

PARASITE:

A general term for any organism that depends on another organism for support, which it receives at the expense of the other organism.

PARASITOLOGY:

A biological discipline devoted to the study of parasites, primarily those among the animal and protist kingdoms. Parasitic bacteria, fungi, and viruses usually are studied within the context of infectious diseases.

PATHOGEN:

A disease-carrying para site, typically a microorganism.

PATHOLOGY:

The study of the essential nature of diseases.

PUBLIC HEALTH:

A set of policies and methods for protecting and improving the health of a community through efforts that include disease prevention, health education, and sanitation.

RNA:

Ribonucleic acid, the molecule translated from DNA in the cell nucleus, the control center of the cell, that directs protein synthesis in the cytoplasm, or the space between cells.

VACCINE:

A preparation containing microorganisms, usually either weakened or dead, which is administered as a means of increasing immunity to the disease caused by those microorganisms.

VECTOR:

An organism, such as an insect, that transmits a pathogen to the body of a host.

Infection control

Microorganisms are easily transmitted from place to place via vectors such as insects or animals, by humans that can harbor the infectious organism and shed them to the environment, and via movement through the air (in the case of some bacteria , yeast , and viruses ). Microorganisms can adapt to antimicrobial treatments (the best example being the acquisition of inheritable antibiotic resistance by bacteria). Thus, the potential for the spread of infection by disease-causing microbes is substantial unless steps are taken to limit the spread. Such strategies are collectively termed infection control.

For many microorganisms, particularly bacteria, contact transmission is a common means of spread of infection. This can involve the fecal-oral route, where hands soiled by exposure to feces are placed in the mouth. Day care workers and the infants under their charge are a significant focus of such Escherichia coli infections. As well, touching a contaminated inanimate surface is a means of transmitting an infectious microorganism.

The contact route of transmission is the most common route in the hospital setting. Various steps can be taken to control the spread of infection through contact with contaminated surfaces. Proper handwashing, in fact, is the single most effective means of preventing the spread of infection. Thorough handwashing prevents spread of bacteria to others and also prevents contamination of work or food preparation surfaces.

The operating theatre is an example of a place where the importance of infection control measures is apparent. In the nineteenth century, before the importance of hygienic procedures was recognized, operations were used as a last resort because of the extremely high mortality rate after surgery. Pioneering efforts by scientists such as Joseph Lister made operating rooms much cleaner, which resulted in a drop in the death rate attributable to surgically acquired infections. In the present day, operating rooms are places where personal hygiene is meticulous, instruments and clothing is sterile, and where post-operative clean up is scrupulous.

In hospitals and particularly in research settings, the control of infections involves the use of filters that can be placed in the ventilation systems. Such filters prevent the movement of particles even as small as viruses from a containment area to other parts of a building. Work surfaces are kept free of clutter and are exposed to disinfectant both before and after work with microbes, to kill any transient organisms that may be on the inanimate surface. Laboratories often contain containment structures called fume hoods, in which organisms can be worked with isolated from the airflow of the remainder of the lab. Even the nature of the work surface is designed to thwart infection. Surfaces are constructed so as to be very smooth and to be watertight. The presence of crevasses and cracks at the junction between surfaces are ideal spots for the collection and breeding of infectious microorganisms.

Some infectious microorganisms can be transferred by animal or insect vectors. One example is the viral agent of Yellow Fever , which is transmitted to humans via the mosquito. Control of such an infection can be challenging. Typically a concerted campaign to kill the breeding population of the vector is required, along with measures to protect people from those vectors that might escape the eradication campaign. To use the Yellow Fever example, spraying in mosquito breeding sites could be supplemented with the use of mosquito netting over the beds of people in particularly susceptible regions.

Another strategy of infection control is the use of antimicrobial or antiviral agents in an effort to either defeat an infection or, in the case of vaccines, to protect against the spread of an infection. Antibiotics are an antimicrobial agent. They have been in common use for less than 75 years, and already history is showing that antibiotics achieve success but that this success should not be assumed to be everlasting. Bacteria are proving to be adept at acquiring resistance to many antibiotics. Indeed, already strains of enterococci and Staphylococcus aureus are known to be resistant to virtually all antibiotics currently in use.

Immunization against infection is a widely practiced and successful infection control strategy. Depending upon the target microbe, the vaccination program may be undertaken to prevent the seasonal occurrence of a malady such as influenza , or to eradicate the illness on a worldwide scale. An example of the latter is the World Health Organization's effort to eradicate polio.

One breeding ground for the development of resistant microbial populations is the hospital. Antibiotics and disinfectants are an important part of the infection control strategy in place in most hospitals. Bacteria are constantly exposed to antibacterial agents. The pressure to adapt is constant.

The degree of infection control is tailored to the institution. For example, in a day care facility, the observance of proper hygiene and proper food preparation may be adequate to protect staff and children. However, in a hospital or nursing home, where people are frequently immunocompromised, additional measures need to be taken to ensure that microbes do not spread. Such measures can include regular disinfection of surfaces, one-time use of specific medical equipment such as disposable needles, and well-functioning ventilation systems.

The focus of infection control strategies has shifted with the emerging knowledge in the 1970s and 1980s of the existence and medical relevance of the adherent bacterial populations known as biofilms. These adherent growths can remain viable on surfaces after being treated with concentrations of chemicals that swiftly kill their free-floating counterparts. Infection control in areas such as physician and dentist offices, now focus on ensuring that equipment is free from biofilms, because the bacteria could be easily transferred from the equipment to a patient.

See also Bacteria and bacterial infection; Disinfection and disinfectants; Epidemics and pandemics; Hygiene

infection The normal human body is covered with billions of harmless microorganisms: indeed, we each carry more bacteria than the total human population of the world! These, together with the skin and the immune system, serve to protect the body from invasion by harmful, or ‘pathogenic’ microorganisms.

If there is a breach in one of these lines of defence, these pathogens can gain access to the body. Entry may be, for example, via a skin wound, inhalation, ingestion, or sexual intercourse, and may be facilitated by immune deficiency or loss of the normal organisms living on the body, for instance after a course of antibiotics.

As soon as the immune system detects the presence of a pathogen it mounts a response to kill it, which is highly successful in most cases in healthy people. On the rare occasions where it fails, or in people with poorly functioning immune systems, the organism may succeed in establishing itself and cause disease: an infection occurs. The term ‘infection’ therefore encompasses not only the classical ‘infectious diseases’, but also such diseases as boils, thrush, urinary tract infection, and surgical wound infections.

The immune response produces a syndrome of inflammation at the site of the infection. This is characterized by redness, warmth, pain, and swelling, caused by extra blood supply to the area bringing white blood cells to fight the infection. Pus may be formed (a mixture of white cells, dead tissue, and organisms). Usually this stops the infection from spreading. However, if the organisms gain entry to the bloodstream, sepsis or ‘blood poisoning’ may ensue. In sepsis the body's white cells respond by producing vast amounts of chemicals which, as well as helping to kill the marauders, result in fever, flushing, shivering, low blood pressure, rapid heart rate, and, in severe cases, delirium. Sometimes this immune response is more harmful than the infection itself. Conversely, sepsis may be difficult to recognize in patients with suppressed immune systems who cannot mount such a florid response. Finally, some microorganisms are not easily recognized by the immune system at all, so that infection may have few if any symptoms until later in the course of the disease when damage to the body by the organism is well advanced. Examples are the human immunodeficiency virus which causes AIDS, and the prion causing Creutzfeld-Jacob disease.

Hospital infection and antisepsis

For many hundreds of years, fevers and infections were believed to be caused by ‘miasmas’, or noxious air exuding from rotten materials. In the nineteenth century the most notorious, and perhaps the most tragic, manifestation of sepsis was puerperal sepsis, or childbed fever, in which the dangerous bacterium Streptococcus pyogenes (now known as the Group A streptococcus) gained entry to the bloodstream via the birth canal. It had a very high fatality rate and was responsible for the deaths of countless young mothers every year. Although well-recognized as a complication of childbirth, the cause was not understood. The Hungarian obstetrician Ignaz Semmelweis, working in Vienna in the 1850s, was particularly concerned by the high rate of childbed fever on one of his wards which was attended by medical students. On this ward nearly a fifth of his patients died of sepsis. On his other ward, attended only by midwives, the rate was only about 3%. He realized that the medical students came directly from the autopsy room to the obstetric ward and proceeded to examine the patients without even washing their hands in between. He insisted that each student should wash his hands with soap and water and then an antiseptic before entering the ward, and saw the mortality rate drop immediately to less than 2%. Thus he proved not only transmission by hand of an infectious agent, but also that it could be prevented by use of antisepsis.

This was a dramatic result, but despite this Semmelweis was ignored and even ridiculed. It was Joseph Lister, working in Glasgow in the late 1860s, who brought about the general acceptance of surgical antisepsis. He used carbolic acid to transform surgery from a highly dangerous last resort to the treatment of choice in many conditions. Florence Nightingale did the same for hospitals after the Crimean War, during which she had shown that cleanliness and hygiene were paramount in preventing injured soldiers from dying of infections — although, ironically, she never believed in the germ theory of disease, rather she believed that filth and dirt bred disease directly.

Since then the refinement of antisepsis before and during operations has been one of the most important developments in allowing the practice of surgery as we now know it. Even now, maintaining a low infection rate is one of the priorities of every surgeon. Low levels are attained by the use of ‘asepsis’ — that is, sterilizing the instruments so that no microorganisms are present on them — and ‘antisepsis’ — the use of chemical solutions to decrease the number of the patient's and the surgeon's own microorganisms as far as possible. Nowadays, one of the greatest challenges facing hospital infection control is the prevention of spread of bacteria that are resistant to many antibiotics, such as methicillin-resistant Straphylococcus aureus (MRSA).

Angharad Puw Davies

infection invasion of plant or animal tissues by microorganisms, i.e., bacteria , viruses , viroids , fungi , rickettsias , and protozoans . The invasion of body tissues by parasitic worms and other higher organisms is commonly referred to as infestation.

Invading organisms such as bacteria produce toxins that damage host tissues and interfere with normal metabolism; some toxins are actually enzymes that, by breaking down host tissues, prevent the localization of infections. Other bacterial substances destroy the host's phagocytes. Viruses and retroviruses are parasitic on host cells, causing cellular degeneration, as in rabies , poliomyelitis , and AIDS , or cellular proliferation, as in warts and cold sores. Some viruses have been associated with the development of certain cancers . Substances produced by many invading organisms cause allergic sensitivity in the host; the immune response to virus infection has been implicated in some diseases (see allergy ).

Infections may be spread via respiratory droplets, direct contact, contaminated food, or vectors, such as insects. They can also be transmitted sexually (see sexually transmitted diseases ) and from mother to fetus. Immunity is the term used to describe the capacity of the host to respond to infection. Drugs that help fight infections include antibiotics and antiviral drugs .

See also specific diseases, diseases of plants .

Bibliography: See J. Waller, The Discovery of the Germ (2003).

infection The invasion of any living organism by disease-causing microorganisms (see pathogen), which proceed to establish themselves, multiply, and produce various symptoms in their host. Pathogens may invade via a wound or (in animals) through the mucous membranes lining the alimentary, respiratory, and reproductive tracts, and may be transmitted by an infected individual, a carrier, or an arthropod vector. Symptoms in animals appear after an initial symptomless incubation period and typically consist of localized inflammation, often with pain and fever. Infections are combated by the body's natural defences (see immune response). Treatment with drugs (see antibiotics; antiseptic) is effective against most bacterial, fungal, and protozoan infections; some viral infections respond to antiviral drugs. See also immunization.

Infection

An infection is an illness caused by microorganisms or bacteria that invade the body. The body's defenses against infections begin with blocking the entry of microorganisms into the system. Hand washing is an effective strategy in preventing the entry of microorganisms into the body through the skin, the respiratory system , or the GI (gastrointestinal ) tract.

Local infections may produce redness, tenderness, and swelling, but systemic infections produce more serious symptoms such as fever, chills, sweats, and fatigue . Many infections will go away on their own, however, as the body's immune system can successfully fight off many infections. Others, however, require treatment, such as the use of antibiotic medications.

see also Immune System.

Catherine Christie

infection (in-fek-shŏn) n. invasion of the body by harmful organisms (pathogens), such as bacteria, fungi, protozoa, rickettsiae, or viruses. The infective agent may be transmitted by a patient or carrier in airborne droplets or by direct contact; by animal or insect vectors; by ingestion of contaminated food or drink; or from an infected mother to the fetus. After an incubation period symptoms appear, usually consisting of either localized inflammation and pain or more remote effects. Treatment with drugs is usually effective against most infections, but there are few specific treatments for many common viral infections.
http://guidance.nice.org.uk/CG2 NICE guidance on infection control

Infection

How Does Infection Occur?

Where Does Infection Occur?

How Do Infections Lead to Illness?

Do Infections Always Cause Illness?

How Do Infections Spread?

What Are the Symptoms of Infection?

What Is the Treatment for Infection?

How Are Infections Prevented?

Resources

Infection is a process in which bacteria, viruses, fungi or other organisms enter the body, attach to cells, and multiply. To do this, they must evade or overcome the body’s natural defenses at each step. Infections have the potential to cause illness, but in many cases the infected person does not get sick.

KEYWORDS

for searching the Internet and other reference sources

Antibiotics

Immunization

Infection

Inflammation

How Does Infection Occur?

Organisms that can cause illness are all around us: in air, water, soil, and food, as well as in the bodies of animals and other people. Infection occurs when some of them get past a series of natural defenses. Those defenses include:

• Skin: The skin physically blocks germs, but may let them in if it is cut or scraped.
• Coughing deeply: This expels germs from the lungs and breathing passages but may be less effective for weak, sick, or injured people.
• Bacteria: Called “resident flora,” harmless bacteria normally are present in some parts of the body. They compete with harmful germs and crowd them out. But they can be weakened or killed by medications, allowing harmful germs to thrive and cause illness.
• Inflammatory response: This is produced by the body’s immune system. Certain kinds of white blood cells-including macrophages and neutrophils-surround and destroy or otherwise attack any kind of germs, often causing fever, redness, and swelling.
• Antibodies: These are proteins produced by the immune system. Some are targeted to attack specific microbes. This response is also called humoral immunity. Usually these antibodies are produced after a person is infected by or exposed to the microbe.

The immune systems responses may fail if the germs are too numerous, or if they are too virulent. “Virulent,” from the Latin for “poisonous,” describes germs that are particularly good at countering the body’s defenses. For instance, some microbes can prevent antibodies from forming against them. Another important factor is the functioning of the immune system. If it is damaged-weakened, for instance, by age or illness-infection is more likely. Babies tend to get more infections because their immune systems have not yet learned to recognize and attack some microbes.

Where Does Infection Occur?

Localized infections

Localized infections remain in one part of the body. Examples include a cut on the hand that gets infected with bacteria, but does not cause problems anywhere else. Localized infections can be very serious if they are internal, such as in the appendix (appendicitis) or in the heart (endocarditis).

Systemic infections

Most serious infections, however, occur when the microorganisms spread throughout the body, usually in the bloodstream. These are called systemic infections, and they include flu, malaria, AIDS, tuberculosis, plague, and most of the infectious diseases whose names are familiar.

How Do Infections Lead to Illness?

The major causes of infection are viruses, bacteria, fungi, and parasites, including protozoa (one-celled organisms), worms, and insects such as mites (which cause scabies) and lice.

Bacteria can release toxins, or poisons. Viruses can take over cells and prevent them from doing their normal work. Bacteria and fungi- and larger infective agents like worms or other parasites-can multiply so rapidly that they physically interfere with the functioning of the lungs, heart, or other organs. The immune response itself-which can bring fever, pain, swelling, and fatigue-often is the major cause of the sick feelings an infected person gets.

Do Infections Always Cause Illness?

No, often they do not. Of people infected with tuberculosis bacteria, for instance, only about one in ten will ever get sick. Some viruses and parasites, too, can remain in the body a lifetime without causing illness. In such cases, called latent infection, people usually get sick only if the immune system weakens.

How Do Infections Spread?

The organisms that cause infections may spread through water, soil, food, or air; through contact with an infected persons blood, skin, or mucus; through sexual contact; or through insect bites. Most germs spread by a couple of these routes; no one microbe spreads in all these ways. In addition, many disease-causing microbes can spread from a pregnant woman to her fetus. When this happens, we say the baby is born with a congenital infection.

What Are the Symptoms of Infection?

The symptoms vary greatly depending on the part of the body and type of organism involved. The first sign of bacterial infection is often inflammation: fever, pain, swelling, redness, and pus. By contrast, viral infections less commonly cause inflammation but may cause a variety of other symptoms, from a runny nose or sore throat to a rash or swollen lymph nodes*.

* lymph nodes

are round masses of tissue that contain immune cells to filter out harmful microorganisms. During infections, lymph nodes may become enlarged.

What Is the Treatment for Infection?

The main treatment is usually medication: antibiotics for bacterial infections; antiviral drugs for some viruses (for most there is no treatment); antifungal medications for fungus infections; and antihelmintic drugs for worms. In some cases of localized infection, as when an abscess or collection of pus forms, surgery may be necessary to drain the infected area.

How Are Infections Prevented?

Disinfeoting wounds

When a wound occurs, infection may be prevented by washing and covering the wound, using antibacterial ointment or spray, and getting medical attention if the wound is serious.

Immunization

Many systemic infectious diseases can be prevented by immunization. Among them are chickenpox, cholera, diphtheria, hepatitis A and hepatitis B, influenza, Lyme disease, measles, mumps, pertussis (whooping cough), pneumococcal pneumonia, polio, rabies, rubella (German measles), tetanus, typhoid fever, and yellow fever.

Hygiene, sanitation, and public health

Many other systemic infections can be prevented by having a clean public water supply and a sanitary system for disposing of human wastes; by washing hands before handling food; by cooking meats thoroughly; by abstaining from sexual contact; and by controlling or avoiding ticks and mosquitos.

See also

Bacterial Infections

Fungal Infections

Parasitic Diseases

Viral Infections

Worms

Resources

U.S. National Institute of Allergy and Infectious Diseases (NIAID), NIAID Office of Communications and Public Liaison, Building 31, Room 7A-50, 31 Center Drive MSC 2520, Bethesda, MD 20892-2520. NIAID publishes pamphlets about infectious diseases and posts fact sheets and newsletters at its website. http://www.niaid.nih.gov/publications/

The World Health Organization posts fact sheets at its website, covering communicable/infectious diseases, tropical diseases, vaccine preventable diseases, and many other health topics. http://www.who.org/home/map_ht.html

KidsHealth.org, the website created by the Nemours Foundation, has information on dozens of infections. http://KidsHealth.org

in·fec·tious / inˈfekshəs/ • adj. (of a disease or disease-causing organism) likely to be transmitted to people, organisms, etc., through the environment. ∎  likely to spread infection: the dogs may still be infectious. ∎  likely to spread or influence others in a rapid manner: her enthusiasm is infectious. DERIVATIVES: in·fec·tious·ly adv. in·fec·tious·ness n.

in·fec·tion / inˈfekshən/ • n. the process of infecting or the state of being infected: strict hygiene will limit the risk of infection. ∎  an infectious disease: a chest infection. ∎  Comput. the presence of a virus in, or its introduction into, a computer system.

infection The invasion of the tissues of a plant by a pathogenic microorganism. Sometimes the term is used as a synonym for a disease caused by a micro-organism.

infection Invasion of the body by disease-causing organisms that become established, multiply and give rise to symptoms.

infectious Applied to a disease that can be transmitted from one individual to another.

infection •ashen, fashion, passion, ration •abstraction, action, attraction, benefaction, compaction, contraction, counteraction, diffraction, enaction, exaction, extraction, faction, fraction, interaction, liquefaction, malefaction, petrifaction, proaction, protraction, putrefaction, redaction, retroaction, satisfaction, stupefaction, subtraction, traction, transaction, tumefaction, vitrifaction •expansion, mansion, scansion, stanchion •sanction •caption, contraption •harshen, Martian •cession, discretion, freshen, session •abjection, affection, circumspection, collection, complexion, confection, connection, convection, correction, defection, deflection, dejection, detection, direction, ejection, election, erection, genuflection, imperfection, infection, inflection, injection, inspection, insurrection, interconnection, interjection, intersection, introspection, lection, misdirection, objection, perfection, predilection, projection, protection, refection, reflection, rejection, resurrection, retrospection, section, selection, subjection, transection, vivisection •exemption, pre-emption, redemption •abstention, apprehension, ascension, attention, circumvention, comprehension, condescension, contention, contravention, convention, declension, detention, dimension, dissension, extension, gentian, hypertension, hypotension, intention, intervention, invention, mention, misapprehension, obtention, pension, prehension, prevention, recension, retention, subvention, supervention, suspension, tension •conception, contraception, deception, exception, inception, interception, misconception, perception, reception •Übermenschen • subsection •ablation, aeration, agnation, Alsatian, Amerasian, Asian, aviation, cetacean, citation, conation, creation, Croatian, crustacean, curation, Dalmatian, delation, dilation, donation, duration, elation, fixation, Galatian, gyration, Haitian, halation, Horatian, ideation, illation, lavation, legation, libation, location, lunation, mutation, natation, nation, negation, notation, nutation, oblation, oration, ovation, potation, relation, rogation, rotation, Sarmatian, sedation, Serbo-Croatian, station, taxation, Thracian, vacation, vexation, vocation, zonation •accretion, Capetian, completion, concretion, deletion, depletion, Diocletian, excretion, Grecian, Helvetian, repletion, Rhodesian, secretion, suppletion, Tahitian, venetian •academician, addition, aesthetician (US esthetician), ambition, audition, beautician, clinician, coition, cosmetician, diagnostician, dialectician, dietitian, Domitian, edition, electrician, emission, fission, fruition, Hermitian, ignition, linguistician, logician, magician, mathematician, Mauritian, mechanician, metaphysician, mission, monition, mortician, munition, musician, obstetrician, omission, optician, paediatrician (US pediatrician), patrician, petition, Phoenician, physician, politician, position, rhetorician, sedition, statistician, suspicion, tactician, technician, theoretician, Titian, tuition, volition •addiction, affliction, benediction, constriction, conviction, crucifixion, depiction, dereliction, diction, eviction, fiction, friction, infliction, interdiction, jurisdiction, malediction, restriction, transfixion, valediction •distinction, extinction, intinction •ascription, circumscription, conscription, decryption, description, Egyptian, encryption, inscription, misdescription, prescription, subscription, superscription, transcription •proscription •concoction, decoction •adoption, option •abortion, apportion, caution, contortion, distortion, extortion, portion, proportion, retortion, torsion •auction •absorption, sorption •commotion, devotion, emotion, groschen, Laotian, locomotion, lotion, motion, notion, Nova Scotian, ocean, potion, promotion •ablution, absolution, allocution, attribution, circumlocution, circumvolution, Confucian, constitution, contribution, convolution, counter-revolution, destitution, dilution, diminution, distribution, electrocution, elocution, evolution, execution, institution, interlocution, irresolution, Lilliputian, locution, perlocution, persecution, pollution, prosecution, prostitution, restitution, retribution, Rosicrucian, solution, substitution, volution •cushion • resumption • München •pincushion •Belorussian, Prussian, Russian •abduction, conduction, construction, deduction, destruction, eduction, effluxion, induction, instruction, introduction, misconstruction, obstruction, production, reduction, ruction, seduction, suction, underproduction •avulsion, compulsion, convulsion, emulsion, expulsion, impulsion, propulsion, repulsion, revulsion •assumption, consumption, gumption, presumption •luncheon, scuncheon, truncheon •compunction, conjunction, dysfunction, expunction, function, junction, malfunction, multifunction, unction •abruption, corruption, disruption, eruption, interruption •T-junction • liposuction •animadversion, aspersion, assertion, aversion, Cistercian, coercion, conversion, desertion, disconcertion, dispersion, diversion, emersion, excursion, exertion, extroversion, immersion, incursion, insertion, interspersion, introversion, Persian, perversion, submersion, subversion, tertian, version •excerption

infectious •factious, fractious •anxious • captious •precious, semi-precious •infectious •conscientious, contentious, licentious, pretentious, sententious, tendentious •Athanasius, audacious, bodacious, cactaceous, capacious, carbonaceous, contumacious, Cretaceous, curvaceous, disputatious, edacious, efficacious, fallacious, farinaceous, flirtatious, foliaceous, fugacious, gracious, hellacious, herbaceous, Ignatius, loquacious, mendacious, mordacious, ostentatious, perspicacious, pertinacious, pugnacious, rapacious, sagacious, salacious, saponaceous, sebaceous, sequacious, setaceous, spacious, tenacious, veracious, vexatious, vivacious, voracious •facetious, Lucretius, specious •adventitious, Aloysius, ambitious, auspicious, avaricious, capricious, conspicuous, delicious, expeditious, factitious, fictitious, flagitious, judicious, lubricious, malicious, Mauritius, meretricious, nutritious, officious, pernicious, propitious, repetitious, seditious, siliceous, superstitious, suppositious, surreptitious, suspicious, vicious •noxious, obnoxious •conscious, subconscious, unselfconscious •cautious, tortious •atrocious, ferocious, precocious •Confucius • luscious •bumptious, scrumptious •compunctious, rambunctious