Viruses Health Article

KEY TERMS

Capsid—The protein structure of a virus.

Capsomere—The protein subunits of the capsid.

Endosome—A membrane-mediated means of transporting materials from outside to inside the cell.

Genome—The genetic material encoding the genes of an organism.

Nucleocapsid—The combination of the capsid and viral genome.

Prion—An unconventional virus that is made almost entirely of protein.

Reverse transcriptase—A retroviral enzyme that produces DNA copies of genetic information encoded by RNA.

Virion—A single infectious virus particle.

Viroid—An unconventional virus that is made of uncoated RNA.

Zoonotic—A type of virus that primarily infects an insect or animal, but can be transmitted to humans.

Prevention

The most effective method of treatment of viral diseases is prevention of the infection. Vaccines, where the immune system is exposed to non-infective viral antigens to allow the development of protective antibodies, have proven effective in controlling many viral illnesses. Vaccines are made of inactivated (killed) virus, attenuated (weakened) virus, or isolated viral proteins, that are known as subunit vaccines. Vaccines are available for the viruses that cause measles, mumps, rubella, poliomyelitis, rabies, hepatitis A and B, influenza, varicella-zoster (chicken pox) and yellow fever. Many other vaccines are in the developmental or clinical trial stages.

The greatest drawback to vaccines is the inability of the protection to counter the same virus that has altered its antigens through mutation. Thus, viruses that undergo rapid mutation are difficult to control using vaccination. One solution used for influenza is to create a new vaccine every season against the viruses that are predicted to be responsible for upcoming flu outbreaks. Although this is an imperfect system, influenza vaccination is instrumental in shortening epidemics and protecting the populations most at risk for complications, including the chronically ill, the elderly, and health care workers (primarily to prevent transmitting infection to those as risk).

A second preventative measure is the avoidance of infection by blocking transmission at the point of viral entry. This is done through the isolation of infected patients and avoiding contact with infected biological material such as lesions, blood, and airborne particles through the use of gloves, masks, and other barriers. Health care providers must practice careful hygiene of patients, including immediate removal of vomit or diarrhea, and thorough hand washing. These measures are taken equally to avoid patient-to-provider and providerto-patient transmission of viruses. For zoonotic viruses, transmission can be reduced through pesticide control of the insect or animal reservoir of the disease.

Common diseases and disorders

Several hundred different viruses infect humans. The viruses that occur chiefly in humans can be categorized as respiratory, enteric, exanthematous, hepatitis, or persistent. The most common respiratory viruses include the rhinoviruses (the common cold) and the influenza viruses. Common enteric viruses include polioviruses (now rare because of vaccination), coxsachie viruses (herpangina), and epidemic gastroenteritis viruses such as rotaviruses. Rubeola (measles) and rubella (German measles) are two common exanthematous viruses.

Hepatitis viruses type A through E are known, with type A most often responsible for epidemics of the disease. Many of the persistent viruses are quite widespread and include cytomegalovirus (usually asymptomatic), Epstein-Barr virus (mononucleosis), Herpes simplex virus (cold sores and genital herpes), human herpes virus type 6 (roseola), human papilloma virus (warts), and varicella-zoster virus (chicken pox and shingles).

Zoonotic viruses, that chiefly infect insects or animals, with humans as minor or accidental host, are generally rarer. The diseases caused by these viruses are limited to areas that can support the insect or animal host as well as humans. Rabies is the most widespread of these diseases. Flaviviruses (yellow and dengue fever), bunyaviruses (California encephalitis and Hantavirus pulmonary syndrome), and filoviruses such as ebola (hemorrhagic fever) are other examples of zoonoitic viruses that cause human disease.

Human disease caused by nonconventional viruses is very rare. The most common is CJD, a prion-mediated disease that occurs in one in a million individuals. Hepatitis D is the only known human viroid, and it requires co-infection with hepatitis B. Other diseases caused by nonconventional viruses are kuru and Gerstmann-Sträussler-Scheinker syndrome (GSS), both caused by prions.

BOOKS

Brooks, George F. et al. "General Properties of Viruses" and "Pathogenesis & Control of Viral Disease." In Medical Microbiology. Appleton & Lange, 1998.

Collier, Leslie and John Oxford. Human Virology. New York: Oxford University Press, 2000.

PERIODICALS

Carpenter, Charles et al. "Antiretroviral Therapy in Adults" JAMA 283 (January 2000): 381–90.

Mahoney, M.C. "Adult immunization—influenza vaccine" American Family Physician 69 (May 2000): 2901–2902.

"Provider-to-Patient Transmission" American Journal of Nursing 99 (May 1999): 17.

OTHER

"Virology." On-line Textbook of Microbiology and Immunology. University of South Carolina School of Medicine, Department of Microbiology. <http://www.med.sc.edu:85/book/welcome.htm> (March 15, 2001).

Michelle L. Johnson, M.S.