Viruses are microorganisms that do not have the ability to independently produce new copies of themselves. Instead, the intact virus or its payload of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) must get inside the host cell. Once inside, copies of the genetic material are made at the same time as the host's genetic material is being replicated, using the various constituents of the host's replication machinery.
Typically, the host cell eventually ruptures, releasing the newly made viruses, which in turn initiate another cycle by infecting other host cells. The host suffers, since cells are being destroyed.
There are many types of viruses that can cause infections in virtually every living thing, including humans.
In general, viral diseases result from the attachment of a virus to the host cell and entry of either the virus particle or its genetic material into the cell. The attachment of a virion to a host involves the interaction of components of the viral and host cell outer surfaces. Often, these components are proteins, but carbohydrates and lipids can be involved. The host component is also known as the receptor. The receptor is not present specifically to allow viral infection. Rather, this host constituent has another function, and the infecting virus has evolved the capability of using this constituent to adhere to the cell surface. As one example, human immunodeficiency virus (HIV), which causes acquired immunodeficiency syndrome (AIDS, also cited as acquired immune deficiency syndrome), uses a host protein called CD4 as the receptor. HIV enters only cells, such as white blood cells, that have this receptor. The entry of the virus kills the white blood cell. Since white blood cells are important in the proper functioning of the host's immune system, their gradual destruction by HIV causes the immune system to break down, leaving the infected person vulnerable to a variety of opportunistic infections and maladies, including some types of cancer.
Following the fusion between the virion and the host cell surface, the virion or the viral genetic material enters the host cell. This fusion can happen in several different ways. An individual virion can enter the host cell in a process called endocytosis—the folding of a portion of the cell surface around the virion. The folding creates a spherical portion of the host cell's surface (a vesicle) that buds off inside the cell. The virion located inside the vesicle is degraded, releasing its genetic material. For other viruses, the viral surface layer melds with the host cell's outer surface, which releases the genetic material inside the host cell. Finally, the viral DNA or RNA can directly enter the host cell, leaving the viral particle stuck to the host cell's surface. An example of this process is the injection of DNA from a bacteriophage into the host cell that it specifically adheres to.
The viral DNA or RNA can then be replicated. In the case of DNA, this replication can occur directly, since the host's genetic material is also DNA. RNA-containing viruses, such as retroviruses (an example is HIV), require an additional step in which the viral RNA is used to make DNA. This is done using an virus-encoded enzyme called reverse transcriptase.
Depending on the virus, the replicated DNA, which is not recognized as foreign by the host's replication machinery, is used to manufacture the proteins that are encoded by the viral genes. The proteins assemble around the copies of replicated genetic material to form the new virus particles that are the hallmark of the viral disease. For other, so-called latent viruses, the replicated genetic material is incorporated into the host's DNA, where it can remain for years until certain conditions—as yet only partially understood—stimulate the viral DNA to excise and begin the production of virus particles. An active infection results. Examples of latent viral infections include AIDS, hepatitis B, Creutzfeld-Jacob disease, and herpes.
WORDS TO KNOW
BACTERIOPHAGE: A virus that infects bacteria. When a bacteriophage that carries the diphtheria toxin gene infects diphtheria bacteria, the bacteria produce diphtheria toxin.
DEGRADED: Any complex chemical that is broken down into less-complex molecules.
DEOXYRIBONUCLEIC ACID (DNA): Deoxyribonucleic acid (DNA) is a double-stranded, helical molecule that forms the molecular basis for heredity in most organisms.
ENDOCYTOSIS: Endocytosis is a process by which host cells allow the entry of outside substances, including viruses, through their cell membranes.
HOST: Organism that serves as the habitat for a parasite, or possibly for a symbiont. A host may provide nutrition to the parasite or symbiont, or simply a place in which to live.
LATENT VIRUS: Latent viruses are those viruses that can incorporate their genetic material into the genetic material of the infected host cell. Because the viral genetic material can then be replicated along with the host material, the virus becomes effectively “silent” with respect to detection by the host. Latent viruses usually contain the information necessary to reverse the latent state. The viral genetic material can leave the host genome to begin the manufacture of new virus particles.
RECEPTOR: Protein molecules on a cells surface that acts as a “signal receiver” and allow communication between cells.
REVERSE TRANSCRIPTASE: An enzyme that makes it possible for a retrovirus to produce DNA (deoxyribonucleic acid) from RNA (ribonucleic acid).
RIBONUCLEIC ACID (RNA): Any of a group of nucleic acids that carry out several important tasks in the synthesis of proteins. Unlike DNA (deoxyribonucleic acid), it has only a single strand. Nucleic acids are complex molecules that contain a cell's genetic information and the instructions for carrying out cellular processes. In eukaryotic cells, the two nucleic acids, ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), work together to direct protein synthesis. Although it is DNA (deoxyribonucleic acid) that contains the instructions for directing the synthesis of specific structural and enzymatic proteins, several types of RNA actually carry out the processes required to produce these proteins. These include messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA). Further processing of the various RNAs is carried out by another type of RNA called small nuclear RNA (snRNA). The structure of RNA is very similar to that of DNA, however, instead of the base thymine, RNA co
VIRION: A virion is a mature virus particle, consisting of a core of ribonucleic acid (RNA) or deoxyribonucleic acid (DNA) surrounded by a protein coat. This is the form in which a virus exists outside of its host cell.
Viruses are classified into a number of families. The members of a given family share similarities in the type of genetic material and its arrangement (either as a double-strand of DNA or RNA or a single strand of the genetic material), chemistry, and physical properties, such as viral size and shape. Members of a given family differ in some characteristics from the members of another family, and different families of viruses cause different diseases.
Parvoviruses are members of the family Parvoviridae. These DNA-containing viruses are small. Their genetic material only codes for three of four proteins. Nonetheless, this small protein armada is sufficient to establish an infection. Diseases caused by parvoviruses include fifth disease, a mild illness characterized by a rash that usually occurs in children.
Papovaviruses are members of the family Papovaviridae. These DNA viruses also have a small genome that encodes five to eight proteins. Examples of papovavirus infections include warts, inflammation of the kidney (nephritis) and the urethra (urethritis), and progressive multifocal leukoencephalopathy. The latter is a disease that causes the loss of a brain component called myelin. The increasing damage to the transmission of nerve impulses is progressively disabling and can be fatal.
The 88 known adenoviruses are members of the family Adenoviridae. These viruses have a distinctive shape that consists of 20 triangular faces (an icosahedron) with long fibers protruding from 12 regions around the viral surface. Adenoviruses cause the common cold, and infections of the liver, bladder (cystitis), eye (keratoconjunctivitis), and gastrointestinal tract (gastroenteritis).
Herpesviruses are members of the family Herpesviridae. These viruses cause latent infections whose symptoms may not appear for years. The eight types of herpesvirus that can be recognized by the immune system cause a variety of infections. These infections include cell damage and the formation of ulcers in the mouth, lips (cold sores), skin, and genitals; keratoconjunctivitis, chickenpox, shingles, two forms of mononucleosis, three types of cancer (Burkitt's lymphoma, orophayngeal carcinoma, and, in AIDS patients, Kaposi's sarcoma), cytomegalovirus infection (which can be fatal in infants), and inflammation of the brain (encephalitis).
Poxviruses are members of the family Poxviridae. The name of the virus refers to the major characteristic of the diseases caused by poxviruses, namely a raised lesion on the skin (pox). Poxviruses are the largest viruses known and may be an intermediate between viruses and bacteria. However, they are still classified as viruses because they are not capable of independent replication. Diseases caused by poxviruses include smallpox, monkeypox, cowpox, and a type of skin infection. But, all poxviruses are not deadly. A poxvirus called vaccinia virus can bestow immunity to smallpox.
Viruses in the families Hepadnaviridae and Coronaviridae cause type B hepatitis and liver cancer, and the common cold, respectively.
Picornaviruses are RNA-containing viruses that are members of the family Picornaviridae. They are the smallest of the RNA viruses, coding for only six to nine genes. Infections caused by picornaviruses include polio, meningitis, the common cold, inflammation of the heart (myocarditis), and inflammation of the tissue surrounding the heart (pericarditis).
Calciviruses are members of the family Calciviridae. There are two members of note. The first is the Norwalk virus, which is notorious for causing disease outbreaks on cruise ships and in crowded communal areas, such as university residences. The virus causes a contagious form of gastroenteritis that is characterized by several days of intense diarrhea and vomiting. The second virus is hepatitis E, which is frequently fatal if contracted by pregnant women. In fact, Hepatitis E is fatal to the woman up to 20% of the time, especially during the third trimester.
Of the more than 150 known types of reovirus, two are significant to humans, causing encephalitis and, most commonly, diarrhea in infants. The latter infection, which is caused by rotavirus, produces dehydration that is fatal if not treated quickly.
Togaviruses are members of the family Togaviridae. Infections caused by togaviruses include rubella (also called German measles), various forms of encephalitis, inflammation of joints (arthritis), and skin inflammation.
Flaviviruses are members of the family Flaviviridae. These viruses cause a number of serious human infections that are transmitted by insects, such as mosquitoes. These infections include yellow fever, dengue, West Nile disease, and encephalitis.
Arenaviruses are members of the family Arenaviridae. The infections caused by arenaviruses are also serious and include hemorrhagic fever and inflammation of the brain and spinal cord or the membranes that cover these regions (lymphocytic choriomeningitis).
Retroviruses, which are members of the family Retroviridae, are given this name because they contain genetic information for the production of an enzyme called reverse transcriptase. The enzyme allows the viral RNA to be used to produce DNA. Retroviruses are important human pathogens, causing cancer of the blood (leukemia) and, most significantly, AIDS.
Orthomyxoviruses include several types of influenza virus, including influenza A (H5N1), which is the cause of avian influenza. Avian influenza is one example of an emerging disease. Other orthomyxoviruses can cause hemorrhagic fever and, along with bunyaviruses, encephalitis. Another viral class, paramyxovirus, includes the virus that causes measles.
Coronaviruses belongs to the family Coronaviridae. A coronavirus of particular significance causes severe acute respiratory syndrome (SARS), and is another example of an emerging disease.
Finally, filoviruses are members of the family Filoviridae. The two known filoviruses are Marburg and Ebola virus, which cause hemorrhagic fevers that can be severe and rapidly lethal. Relatively little is known about the infectious disease processes of these viruses or of their natural hosts because they are so dangerous to work with (a special containment facility called a biosafety level 4 laboratory is required for research on these microbes) and because illness outbreaks appear sporadically and end quickly.
Reflecting the diversity of viruses and viral diseases, treatment is variable. One common characteristic is the ineffectiveness of antibiotics, since antibiotics are effective against bacteria. Treatment strategies include blocking the attachment of the virus to host cells by occupying the host cell receptor with another molecule, use of a vaccine that has been developed for a particular virus or a similar target of different viruses, and taking medications that assist the immune system in responding to the presence of the infecting virus.
Likewise, prevention strategies vary. Wearing a protective mask to prevent inhalation of viral-contaminated droplets is mandated for healthcare providers when treating someone known or suspected of having SARS, for example. Protective gowns and gloves can be prudent measures when coming into contact with someone who has a hemorrhagic fever, since splashing or copious loss of blood can occur. Another viral barrier is the condom. Wearing a condom can lessen the risk of transmission of HIV during sexual intercourse. Complete abstinence from sex is the ultimate preventative strategy for sexually-transmitted viral disease, although this strategy can be difficult to follow in everyday life.
The incidence of viral diseases, such as West Nile disease, that are transmitted by insects can be reduced by eradicating insect breeding grounds and wearing clothing that protects the body from insect bites. Insect repellents are also a helpful preventative measure.
The toll from viral diseases throughout history is incalculable. The death toll from viral gastroenteritis and measles exceeds 2 million each year, according to the World Health Organization (WHO). In the past 40 years, AIDS has grown in scope from a handful of cases to over 40 million cases and almost 3 million deaths in 2006.
The tragedy of many viral diseases is their concentration in poorer regions of the world, where access to health care and personal living conditions are not as good as in developed countries, such as the United States. AIDS, for example, exacts a huge toll in sub-Saharan Africa. It is common to find villages populated mainly by pre-adolescents and the elderly, with the generations from 20–60 having been decimated. Aside from the human tragedy, the massive loss of the majority of productive wage earners is economically devastating. Many African nations have been economically crippled and little relief is foreseen for generations.
One tragic aspect of the viral disease situation in under-developed and developing countries is that treatments, including vaccines, exist for some of these diseases, but they are not readily available or affordable in these countries. Distribution of the needed medical supplies has been and continues to be driven mainly by humanitarian initiatives of organizations such as the WHO, UNICEF, and the U.S. Centers for Disease Control and Prevention, rather than by commercial interests.
Finally, several of the highest profile emerging diseases are viral diseases. Ebola, avian influenza, and SARS are examples of diseases that have emerged as problems only relatively recently. Of these, avian influenza is a particular concern, since the virus that causes this disease may have developed the ability to spread directly from person to person, instead of only spreading from poultry to humans. This evolution combined with the expanding geographical range of the disease has raised concerns that this virus could cause a worldwide epidemic of a serious and frequently lethal form of influenza.
See AlsoAIDS (Acquired Immunodeficiency Syndrome); Antiviral Drugs; Arthropod-borne Disease; Avian Influenza; B virus (Cercopithecine herpesvirus 1) Infection; CMV (Cytomegalovirus) Infection; Eastern Equine Encephalitis; Ebola; H5N1 Virus; Hepatitis A; Hepatitis B; Hepatitis C; Hepatitis D; Hepatitis E; Influenza Pandemic of 1918; Influenza Pandemic of 1957; Influenza, Tracking Seasonal Influences and Virus Mutation; Influenza; Measles (Rubeola); Monkeypox; Mononucleosis; Mosquitoborne diseases; Mumps; Nipah Virus Encephalitis; Norovirus Infection; Polio (Poliomyelitis); Polio Eradication Campaign; Rabies; Retroviruses; Rotavirus Infection; RSV (Respiratory Syncytial Virus) Infection; SARS (Severe Acute Respiratory Syndrome); St. Louis Encephalitis; West Nile.
Tabor, Edward. Emerging Viruses in Human Populations. New York: Elsevier, 2007.
The Writing Committee of the World Health Organization Consultation on Human Influenza A/H5. “Avian Influenza A (H5N1) Infection in Humans.” New England Journal of Medicine 353 (September 29, 2005): 1374–1385.