COMMUNICABLE DISEASE CONTROL

A communicable disease is an illness caused by a specific infectious agent or its toxic products. It arises through transmission of that agent or its products from an infected person, animal, or inanimate reservoir to a susceptible host, either directly or indirectly (through an intermediate plant or animal host, vector, or the inanimate environment). Control of disease is the reduction of disease incidence, prevalence, morbidity, or mortality to a locally acceptable level as a result of deliberate efforts; continued intervention measures are required to maintain the reduction. Control is to be contrasted with elimination (reduction to zero of the incidence of a specified disease in a defined geographic area as a result of deliberate efforts; continued intervention measures are required), eradication (permanent reduction to zero of the worldwide incidence of infection caused by a specific agent as a result of deliberate efforts; intervention measures are no longer needed), and extinction (the specific infectious agent no longer exists in nature or the laboratory).

Communicable diseases may be classified according to the causative agent, the clinical illness caused, or the means of transmission. Often all three characteristics are used (e.g., food-borne Salmonella gastroenteritis). Causative agents include bacteria, viruses, and parasites. Examples of bacterial diseases include pneumococcal pneumonia and gonorrhea. Viral diseases include influenza, measles, and ebola. Parasitic diseases include malaria and schistosomiasis. Other communicable diseases may be caused by other types of microorganisms such as fungi (e.g., histoplasmosis). The types of illness include pneumonia, diarrhea, meningitis, or other clinical syndromes.

Various categorizations of means of transmission have been used. The American Public Health Association uses these categories: direct transmission, indirect transmission, and airborne. Direct transmission refers to direct contact such as touching, biting, kissing, or sexual intercourse, or the direct projection of droplet spray into the eye, nose, or mouth during sneezing, coughing, spitting, singing, or talking. This projection usually is limited to a distance of 1 meter or less. Examples of direct contact transmission include rabies and sexually transmitted HIV (human immunodeficiency virus). Direct projection is responsible for transmission of diseases such as measles and influenza.

Indirect transmission may occur through a vehicle or an arthropod vector. The causative agent may or may not multiply or develop in or on the vehicle. Examples of possible vehicles include water, food, biological products, or contaminated articles (such as syringe needles). Water-and foodborne diseases have the potential for causing outbreaks involving thousands of persons. Before the causative agent was identified, many cases of HIV resulted from blood transfusion. Since all donor blood in the United States is now screened for HIV, this is no longer a significant means of transmission. However, sharing of needles by injection drug users remains an important factor in the AIDS (acquired immunodeficiency syndrome) epidemic. Arthropod vectors can spread disease mechanically (as a result of contamination of their feet or passage of organisms through the gastrointestinal tract) or biologically (in which the agent must multiply or go through one or more stages of its life cycle before the arthropod becomes infective). Mechanical spread by arthropod vectors is uncommon. However, arthropod-borne diseases such as malaria (in which the parasite develops within the mosquito vector) are still responsible for millions of cases and hundreds of thousands of deaths each year in tropical countries.

Some infectious agents can be spread through the air over long distances. Airborne spread requires that infectious particles are small enough to be suspended in the air and inhaled by the recipient. Tuberculosis and histoplasmosis are bacterial and fungal diseases spread in this fashion. Airborne transmission could also be used to disseminate agents of biological warfare or bioterrorism. Anthrax and smallpox have been considered among the most likely biological weapons.

Diseases of animals that can be spread to humans are called zoonoses. Some zoonotic diseases include rabies, plague, and tularemia (rabbit fever).

METHODS OF CONTROL

Communicable diseases occur only when the causative agent comes into contact with a susceptible host in a suitable environment. Prevention and control efforts for communicable diseases may be directed to any of these three elements. Communicable diseases affect both individuals and communities, so control efforts may be directed at both. Treatment of persons with communicable diseases with antibiotics typically kills the agent and renders them noninfectious. Thus, treatment is also prevention. A simple way to prevent the occurrence of communicable diseases is to eliminate the infectious agent through, for example, cooking food, washing hands, and sterilizing surgical instruments between use. Assuring the safety of drinking water through filtration and chlorination and treating sewage appropriately are other important means of preventing the spread of communicable diseases.

For most communicable diseases there is an interval between infection and occurrence of symptoms (the incubation period) in which the infectious agent is multiplying or developing. Some persons who are infected may never develop manifestations of the disease even though they may be capable of transmitting it (inapparent infection). Some persons may carry (and transmit) the agent over prolonged periods (carriers) whether or not they develop symptoms. Treatment during the incubation period may cure the infection, thereby preventing both disease and transmission. This preventive treatment (chemoprophylaxis) is often used in persons who have been exposed to sexually transmitted diseases such as syphilis and gonorrhea. It also is effective in persons who have been infected with tuberculosis, although the preventive treatment must be given for several months.

The susceptibility of the host to a specific infectious agent can be altered through immunization (e.g., against measles) or through taking medications that can prevent establishment of infection following exposure (chemoprophylaxis). Since malnutrition and specific vitamin deficiencies (such as vitamin A) may increase susceptibility to infection, ensuring proper nutrition and administering vitamin A can be more general ways of increasing host resistance. If persons survive a communicable disease, he or she may develop immunity that will prevent the disease from recurring if re-exposed to the causative agent.

The environment may be rendered less suitable for the occurrence of disease in a variety of ways. For example, food can be kept hot or cold (rather than warm) to prevent multiplication of organisms that may be present. Individuals can use mosquito repellents or mosquito nets to prevent being bitten by infected mosquitoes. Breeding places can be drained or insecticides used to eliminate vectors of disease. Condoms can be used to prevent sexually transmitted diseases by providing a mechanical barrier to transmission. Reduction of crowding and appropriate ventilation can reduce the likelihood of droplet or airborne transmission. Respiratory protective devices can be used to prevent passage of microorganisms into the respiratory tract.

The sociocultural environment is also important in affecting the occurrence of communicable diseases. For example, in the 1980s there was a change in the social norms in men who have sex with other men on the West Coast of the United States, where unprotected anal intercourse had been the norm and was responsible for considerable transmission of HIV. As a result of a variety of educational and social marketing approaches, the social norm changed to the use of condoms and the rate of new HIV infections (and of rectal gonorrhea) declined. Similarly, aggressive social marketing of condom use in Uganda has led to a change in sexual practices and a decline in new HIV infection rates. Other societal approaches to control of communicable diseases include safe water and food laws, provision of free immunization and chemoprophylaxis through public health departments, enactment and enforcement of school immunization requirements, isolation of individuals with communicable diseases to prevent transmission, and quarantine of individuals exposed to communicable diseases to prevent disease transmission during the incubation period if they have been infected.

IMPACT OF COMMUNICABLE DISEASES

The gathering of humans in settlements (and subsequently cities) resulted in the development of periodic epidemics of communicable diseases, often with devastating impact. In the fourteenth century, for example, bubonic plague (carried by rats and transmitted to humans by fleas) swept through Europe, killing approximately one-quarter of the population of the continent. Epidemics of "crowd" diseases such as measles and influenza resulted from person-to-person transmission, and inadequate water and sewage management led to epidemics of diseases such as cholera and typhoid. Milk-and food-borne diseases also were common. Until the end of the nineteenth century, communicable diseases were the leading cause of death throughout the world.

In the United States in 1900, tuberculosis was the leading cause of death, followed by pneumonia and diarrhea. Along with diphtheria (in tenth place), these conditions accounted for more than 30 percent of all deaths in the country. Major reductions in morbidity and mortality from communicable diseases have resulted from improvements in sanitation, housing, and nutrition as well as introduction and use of vaccines and specific therapies.

Improvements in sanitation have dramatically reduced the burden of water-and food-borne diseases. Improvements in housing have also played an important role in reducing transmission of tuberculosis, and improvements in nutrition have made persons with infectious diseases less likely to die from their infections. The introduction and use of vaccines have resulted in global eradication of smallpox, significant progress toward eradication of poliomyelitis, and a marked reduction in illness and death due to diseases such as diphtheria, whooping cough (pertussis), and measles. Specific therapies such as antibiotics and antiparasitic drugs have had a significant impact on deaths due to infectious diseases as well as having some impact on the occurrence of the diseases by shortening the period in which an infected person is infectious to others.

The most dramatic improvements have been seen in the United States and other developed nations (see Figure 1). Although significant progress has also been made in developing nations, the World Health Report 2000 reports that 14 million deaths (25 percent of all deaths in the world in 1999) resulted from infectious diseases or their complications. There is a marked disparity in the importance of infectious diseases in high-income countries compared to middle-and low-income countries. In high-income countries, infectious diseases accounted for only 6 percent of all deaths, whereas in middle-and low-income countries they accounted for 28 percent of all deaths.

Worldwide, lower respiratory infections (e.g., pneumonia) and diarrhea are the leading infectious causes of death; each of these conditions can be caused by a variety of microorganisms. AIDS

Figure 1

was the single leading infectious cause of death in 1998, with an estimated 2.2 million deaths, followed by tuberculosis, with nearly 1.5 million deaths, and malaria, with 1.1 million deaths. Nearly 900,000 children died as a result of measles in 1998, even though an effective vaccine against measles was introduced in 1963 and has had a major impact in developed nations. Half of the children who died from measles lived in sub-Saharan Africa.

Much of the continuing toll of communicable diseases could be reduced by more effective use of existing vaccines and other tools for control of infectious diseases. For example, more effective use of measles vaccine and administration of vitamin A could prevent most of the deaths from measles. More widespread use of oral rehydration therapy in diarrhea (to combat the dehydration that is one of the major causes of death) could dramatically reduce current mortality. More effective use of bed nets, anti-mosquito strategies, and appropriate treatment could dramatically reduce malaria deaths. However, new tools will be needed to bring about maximum control of some diseases. Because microorganisms are continually evolving, they may change enough so that prior experience (infection) with the infectious agent does not provide protection. For example, influenza viruses may undergo dramatic changes with the result that pandemics (worldwide epidemics) may occur. In 1918–1919, pandemic influenza killed millions of people worldwide, more than 500,000 in the United States alone (see Figure 1).

PREVENTIVE MEASURES

Vaccine-Preventable Diseases. Some communicable diseases can be prevented by the use of vaccines. The word vaccine comes from vaccinia, the Latin name for cowpox. The first vaccine was developed by Edward Jenner, an eighteenth-century English physician and naturalist who noticed that milkmaids who had acquired cowpox (a condition that caused lesions to appear on the udders of cows) on their hands did not seem to be affected by smallpox. He believed that infection with cowpox would protect against smallpox, a serious, often fatal epidemic disease. In 1796 he took material from a skin lesion on the hand of a milkmaid and inoculated it into the arm of a young boy. The boy was subsequently exposed to smallpox and did not become ill. Thus began the vaccine era.

It was nearly one hundred years until the next vaccine (rabies) was developed by Louis Pasteur. In the twentieth century, a number of vaccines were developed; many more are under development as a result of the biotechnology revolution. Widespread use of vaccines in children has had a dramatic impact on the occurrence of the diseases.

Because smallpox has been eradicated, smallpox vaccination is no longer carried out. The last case of naturally occurring smallpox in a human was in 1977, and in 1980 the World Health Assembly certified that smallpox had been eradicated from the face of the earth. Stocks of smallpox virus have been maintained (under security) in both the United States and Russia, though the debate continues whether they should be destroyed. Concerns have arisen about the possibility that some groups or nations have retained the smallpox virus and developed it for use in biological warfare or bioterrorism.

Chemoprophylaxis. Chemoprophylaxis refers to the practice of giving anti-infective drugs to prevent occurrence of disease in individuals who are likely to be exposed to an infectious disease or who might have already been infected but have not developed disease. For example, individuals traveling to areas where malaria is common can take anti-malarial drugs before arriving, during their stay, and for a few weeks after leaving and thus protect themselves against malaria. Similarly, persons who have been exposed to syphilis can be given penicillin to prevent the possibility of their developing syphilis, and persons who have been infected with tuberculosis can be given six months of treatment to prevent the development of tuberculosis.

Antibiotics and Resistance. Antibiotics are compounds that are produced by microorganisms that kill or inhibit the growth of other microorganisms. Those that kill bacteria are called bactericidal; those that prevent multiplication (and rely on the body's defense mechanisms to deal with the limited number of living organisms) are called bacteriostatic. Some antibiotics are effective against a limited number of microorganisms, others may have more widespread effect.

Because microorganisms are continually in a state of evolution, strains may evolve that are resistant to a particular antibiotic. In addition, resistance characteristics can be transferred from some microorganisms to others (this is particularly true of organisms that inhabit the gastrointestinal tract). The likelihood that resistance will develop is increased if antibiotics are used in an indiscriminate manner and in inadequate amounts (either in terms of individual dosage or in length of therapy). Antimicrobial resistance is a growing problem: organisms that once were exquisitely sensitive to a particular antibiotic may now have developed significant (or total) resistance to it. This necessitates either increasing the dose of the antibiotic administered (in the case of partial resistance) or developing totally new drugs to treat the infection (in the case of total resistance). A few microorganisms (such as enterococcus, an organism that lives in the intestinal tract and is particularly likely to cause infections in gravely ill patients with compromised immune systems) have developed such widespread resistance that it is a real challenge to treat them effectively, resulting in a need to develop even more antibiotics.

EMERGING AND RE-EMERGING INFECTIOUS DISEASES

New infectious diseases continue to be recognized and others, once thought under control, are reemerging as significant problems. To cite a few examples of "new" diseases, the following have been recognized for the first time since 1975: legionnaire's disease, ebola virus, HIV/AIDS (acquired immunodeficiency syndrome), toxic shock syndrome, Escherichia coli O157:H7 (cause of hemolytic-uremic syndrome), Lyme disease, Helicobacter pylori (major cause of peptic ulcer), hepatitis C, and hantavirus. Some of these are conditions previously known but without a known infectious cause (e.g., peptic ulcer) while others represent apparently new clinical syndromes that have not occurred or have not been recognized in the past.

Old diseases, such as tuberculosis and malaria, are reemerging in areas where they were once under control. This may be a result of the lack of continued application of known effective interventions but also may result from ecological changes. Some of the factors involved in the increase in infectious diseases, whether new or old, include population shifts and growth (and encroachment on previously unpopulated areas); changes in behavior (e.g., injection drug use, sexual practices); urbanization, poverty, and crowding; changes in ecology and climate; evolution of microbes; inadequacy of the public health infrastructure to deal with the problems; modern travel and trade; and the increasing numbers of persons with compromised immune systems (whether as a result of HIV/AIDS, chemotherapy for cancer, or immunosuppresive therapy for organ transplants). Many of these factors are interrelated.

In addition to these new and reemerging diseases, there may be specific interactions between diseases. This is particularly true with HIV and tuberculosis (TB), in which each infection is a very potent co-factor for worsening the other: Persons with HIV infection who become infected with TB are more likely to develop TB disease that is serious and rapidly progressive than persons without HIV infection, and persons with TB who contract HIV infection are very likely to have a rapid progression to full-blown AIDS.

In the United States, the incidence of foodborne disease has received increasing attention in the past several years. This relates in part to improved surveillance but also relates to changes in patterns of food production, distribution, and consumption. With modern transportation, it is possible to get fresh vegetables and fruits at all times of the year. This means that salad ingredients purchased at a modern supermarket (and eaten raw) may have been grown in a developing country, where the average American traveler would not eat raw vegetables. The consolidation of producers of prepared foods makes possible large interstate outbreaks of food-borne disease such as the 1994 outbreak of Salmonella infections associated with ice cream that affected an estimated 224,000 persons nationwide. It is currently estimated that food-borne diseases cause approximately 76 million illnesses, 325,000 hospitalizations, and 5,000 deaths in the United States each year.

EPIDEMIC THEORY AND MATHEMATICAL MODELS OF INFECTIOUS DISEASES

Based on observed characteristics of infectious diseases, epidemiologists have attempted to construct mathematical models that would make it possible to predict the pattern of spread of a condition within the population. Some diseases have constant features, which make mathematical modeling particularly attractive. Measles, for example, has a predictable incubation period (ten to fourteen days) and limited duration of infectivity of a given patient (four to seven days). In addition, it is highly infectious (nearly every susceptible person who comes in contact with an infectious person will become infected), and nearly everyone who is infected develops clinical illness. Lifelong immunity follows infection. There is no nonhuman reservoir. Given these relatively constant parameters, it is possible to predict the pattern of transmission if measles is introduced into a population, using different estimates for the proportion of susceptible persons in the population, the distribution of these susceptibles (e.g., randomly dispersed, clustered together), and the likelihood of contact between the infectious patient and the susceptibles. Because of the extreme infectiousness of measles, models indicate that it is necessary to reach very high levels of immunity in a population (on the order of 95 percent or greater) in order to prevent sustained transmission of measles. Given the fact that measles vaccine is approximately 95 percent effective, this indicates that, to eradicate measles, it will be necessary to reach 100 percent of the population with a single dose of the vaccine or to reach 90 percent of the population on each of two rounds of vaccination (assuming that the second round will reach 90 percent of those who were not reached by the first round). Since babies are being born all the time, this also must be an ongoing process. The major reason for continuing debate over whether measles eradication is an achievable goal using current vaccines is the necessity to achieve and maintain such high levels of immunity.

Alan R. Hinman

(see also: Emerging Infectious Diseases; Food-Borne Diseases; Immunizations; Sexually Transmitted Diseases; Vector-Borne Diseases; Waterborne Diseases; Zoonoses; and articles on specific diseases mentioned hereine )

Bibliography

Centers for Disease Control and Prevention (1999). "Achievements in Public Health, 1900–1999: Control of Infectious Diseases." Morbidity and Mortality Weekly Report 48 (29):621–629.

Chin, J., ed. (1999). Control of Communicable Diseases Manual, 17th edition. Washington, DC: American Public Health Association.

Goodman R. A.; Foster, K. L.; Trowbridge, F. L.; and Figueroa, J. P. (1998). "Global Disease Elimination and Eradication as Public Health Strategies." Bulletin World Health Organization 76 (Supp. 2):1–162.

Hinman, A. R. (1998). "Global Progress in Infectious Disease Control." Vaccine 16 (11/12):1116–1121.

Wallace, R. B., ed. (1998). Maxcy-Rosenau-Last Public Health and Preventive Medicine, 14th edition. Stamford, CT: Appleton and Lange.

World Health Organization (1999). The World Health Report 1999. Geneva: Author.

COMMUNICABLE DISEASES

Measles

Measles, a childhood disease characterized by high fever, sore throat, and skin rash, was widespread in the 1920s, but was not usually fatal when patients received good care. However, in foundling hospitals half the patients might die from terminal bronchopneumonia. There was also the danger of developing blindness. Although the microorganism, or "germ," that caused measles had not been indentified, a serum made from the blood of convalescent patients was used after 1920 to provide some resistance to the disease for children who were exposed to measles. It was not completely effective in immunizing the exposed children, but those who became infected usually had a lighter case.

Scarlet Fever

Before 1923 scarlet fever was a danger faced by children and adults on a daily basis. Through the work of a husband-and-wife scientific team, the germ responsible for the disease was recognized, and an inoculation was created to prevent its deadly complications. By 1924 there was still no cure for scarlet fever, but new preventive measures introduced by George and Gladys Dick removed the danger of a disease that might cause deafness, blindness, heart and kidney disease, permanent crippling, or death.

SCARLET FEVER

Emily Breeden, who had scarlet fever when she was a little girl in Marlboro County, South Carolina, recalled how the quarantine program worked in the early 1920s before George and Gladys Dick developed the antitoxin in 1923. The county board of health posted a yellow flag in Emily's front yard and a printed notice on the door to ward off any visitors. The patient and her mother stayed in one room while another member of the family left food on a tray outside the door. The mother carefully washed the dishes with disinfectant before placing them outside the door again. Confinement lasted one month, and then the room had to be fumigated. Emily carefully hung the homework papers she had prepared for school on a clothesline tied across the room so that they could be fumi-gated, too. After she and her mother left the room at the end of the quarantine period, rags were stuffed around the windows and doors to make the room as airtight as possible, and a quart of formaldehyde was poured into a fourteen-quart container with thirteen and one-half ounces of permanganate potash. The resulting fumes were supposed to kill any scarlet fever germs that remained in the room, including those on the homework papers, which Emily then took to school.

Source:

Suzanne C. Linder, Medicine in Marlboro County, 1736-1980 (Baltimore: Gateway Press, 1980), pp. 75-76.

Research Accelerates

Although research into the diseases of typhoid and diphtheria had proved fruitful, little was known about scarlet fever despite the great amount of work that was being done in that area. In order to remedy this situation, the John McCormick Institute for Contagious Diseases was founded by Mr. and Mrs. Harold McCormick of Chicago in honor of the son they lost to scarlet fever. The Dicks went to work trying to identify the scarlet fever microbe.

Identifying the Germ

In 1923 the germ hemolytic streptococcus was definitely identified as the cause of scarlet fever. The germ was isolated from the sore on the finger of a nurse with the disease and then swabbed on the tonsils of several volunteers. When a typical case of scarlet fever resulted, the Dicks deemed the experiment conclusive.

Developing the Antitoxin

Experimentation with the newly discovered germ showed that when subjects were injected with a diluted version, a reaction would appear if that subject had never had the disease. This meant that the subject was susceptible to contracting scarlet fever. Subjects who had had the disease showed no reaction at all. Thus, the Dick test became a conclusive method of determining susceptibility or immunity to the disease. Finally, when larger amounts of the scarlet fever toxin were injected into subjects previously showing a positive reaction to the Dick test, the skin test became negative and the subject was now immune to the disease. The scarlet fever inoculation was born.

Prevention, Not Cure

These experiments in the 1920s virtually eliminated the threat of scarlet fever in epidemic proportions. The Dick skin test and the immunization that followed were not cures for scarlet fever, but preventive measures. Nevertheless, as long as people were willing to be tested and immunized if that test was positive, they were protected from the ravages of the disease.

Tuberculosis

In 1922 Census Bureau compilations showed that 90,452 people in the United States died of tuberculosis, a deadly and contagious disease caused by the bacterium Myco bacterium tuberculosisy which was first identified in 1882 by Robert Koch. Although the disease can involve almost any organ or tissue of the body, between 92 percent and 94 percent of infection is pulmo-nary. The most common mode of transmission is by inhalation of bacilli from the sputum of persons with ulcerative pulmonary tuberculosis. Minute droplets discharged by cough or sneeze from the infected person may float in the air for hours.

Skin Test

Since tuberculosis is easily spread and can be deadly, scientists realized early the necessity of identifying unknowing carriers of the disease. Thus, the first step in limiting the spread of tuberculosis was taken in 1890 when Koch developed the tuberculin skin test. The test, if positive, resulted in a reddened, inflamed patch when small amounts of tuberculin were injected beneath the skin. Tuberculin was a chemical released from the tuberculosis bacterium that caused an allergic reaction (the inflamed patch of skin) in tuberculosis-infected individuals.

Mortality Rates

The tuberculin skin test was a valuable tool in diagnosing victims of tuberculosis, but the mortality rate from the disease remained high in the 1920s. The death rate in 1922 was 97 per 100,000 population. Colorado showed the highest rate at 172.6 per 100,000, probably because infected persons migrated there to take advantage of the cool, dry climate. Nebraska had the lowest rate at 36.1.

Vaccine

Although the death rate from tuberculosis was 2.4 percent lower in 1922 than in 1921, merely identifying carriers of the disease was not enough to stop its progression. The most significant development in reducing the tuberculosis death count came in 1921 when two French microbiologists, Albert Calmette and Camille Guerin, produced the first vaccine. Calmette was a student of Louis Pasteur in Paris, and Guerin was a veterinarian who joined Calmette to study the microbiology of infectious diseases, particularly tuberculosis. The research of the two scientists showed that exposure to tuberculosis or suffering a mild infection of the disease led to eventual resistance. This resistance was caused by the immune system's response to the bacteria in the body.

Calmette and Guerin

From 1906 to 1921 Calmette and Guerin cultured the tuberculosis bacteria from cattle and found that the bacteria lost ability to cause the disease over many generations. Despite their weakened form, these harmless bacteria were able to stimulate the cow's immune system to produce antibodies and protect against the disease.

BCG

Although there was some concern regarding the vaccine's transference to humans, Calmette and Guerin produced a harmless vaccine in 1921, a strain called Bacillus Calmette-Guerin (BCG). The immunization was harmless because the avirulent strain could not damage lung tissue. The vaccine was used in Paris in 1922 and throughout Europe and Asia by 1930. The United States and England were less receptive to the vaccine and insisted on extensive testing before beginning BCG immunizations in the 1950s. With worldwide acceptance, the incidence of tuberculosis declined.

U.S. Response

The vaccine was controversial in the United States because it used specially bred live bacteria, and it conflicted with the widely used skin test as developed by Koch. The skin test was designed to identify carriers of the disease for treatment, but anyone vaccinated showed a positive skin test even when not infected. Nevertheless, the vaccine was eventually accepted worldwide by the 1950s, and tuberculosis was finally reduced to a disease that could be prevented, or identified and treated.

Sources:

Barbara Bates, Bargaining for Life: A Social History of Tuberculosis, 1876-1938 (Philadelphia: University of Pennsylvania Press, 1992);

George F. Dick and Gladys H. Dick, "The Etiology of Scarlet Fever," Journal of the American Medical Association (JAMA), 82 (26 January 1924): 301-302;

Dick and Dick, "Scarlet Fever," American Journal of Public Health, 14 (December 1924): 1022-1028;

Ernest Graening, "Another Germ Bites the Dust," Colliers, 74 (4 October 1924); 26;

"The Mortality From Tuberculosis and Cancer," Science, 58 (13 July 1923): 510;

Herbert T. Wade, ed., The New International Year Book for 1926 (New York: Dodd, Mead, 1927), p. 459;

Gene H. Stollerman, "The Historical Role of the Dick Test," JAMA, 250 (9 December 1983): 3097-3099.

communicable diseases illnesses caused by microorganisms and transmitted from an infected person or animal to another person or animal. Some diseases are passed on by direct or indirect contact with infected persons or with their excretions. Most diseases are spread through contact or close proximity because the causative bacteria or viruses are airborne; i.e., they can be expelled from the nose and mouth of the infected person and inhaled by anyone in the vicinity. Such diseases include diphtheria, scarlet fever, measles, mumps, whooping cough, influenza, and smallpox. Some infectious diseases can be spread only indirectly, usually through contaminated food or water, e.g., typhoid, cholera, dysentery. Still other infections are introduced into the body by animal or insect carriers, e.g., rabies, malaria, encephalitis, Rocky Mountain spotted fever. The human disease carriers, i.e., the healthy persons who may be immune to the organisms they harbor, are also a source of transmission. Some infective organisms require specific circumstances for their transmission, e.g., sexual contact in syphilis and gonorrhea, injury in the presence of infected soil or dirt in tetanus, infected tranfusion blood or medical instruments in serum hepatitis and sometimes in malaria. In the case of AIDS, while a number of different circumstances will transmit the disease, each requires the introduction of a contaminant into the bloodstream. A disease such as tuberculosis may be transmitted in several ways—by contact (human or animal), through food or eating utensils, and by the air. Control of communicable disease depends upon recognition of the many ways transmission takes place. It must include isolation or even quarantine of persons with certain diseases. Proper antisepsis (see antiseptic ) should be observed in illness and in health. Immunologic measures (see immunity ) should be utilized fully. Some sexually transmitted infections are associated with cancer (cervical or penile). Education of the population in rules of public health is of great importance both in the matter of personal responsibility (disposal of secretions, preventing contact with the blood of others, proper handling and preparation of food, personal hygiene) and community responsibility (safe water and food supply, sterile blood supply, garbage and waste disposal). Animal and insect carriers must be controlled, and the activities of human carriers must be limited.

communicable disease (contagious disease, infectious disease) (kŏ-mew-nik-ăbŭl) n. any disease that can be transmitted from one person to another. This may occur by direct physical contact, by common handling of a contaminated object (see fomes), through a disease carrier, or by spread of infected droplets exhaled into the air.