Viral Diseases

In order to understand viral infections, one must understand a little about how a virus functions. All viruses are obligate parasites; that is, they de­pend on a “host” to survive and reproduce. In the case of a virus, the host is the cell of a living organism. Outside of a host cell, viruses are inert mol­ecules, waiting to attach to a victim cell. Although viruses contain genetic material (either deoxyribonucleic acid [DNA] or ribonucleic acid [RNA]), they lack the internal machinery (organelles) to produce proteins from the information contained in their own genetic code. The simplest virus con­tains only three genes.

A viral infection begins when a virus inserts its genetic material into a host cell. First, the virus attaches to a specific structure on the cell’s surface via an attachment protein. Depending on the virus, either the genetic ma­terial diffuses into the host cell or the entire virus enters the cell. The po­liomyelitis virus may have over one million copies of its basic genetic information (RNA) inside a single, infected human intestinal mucosal cell.

One or more of the genes on the viral genetic material code for en­zymes that essentially “hijack” the host cell, causing it to produce only vi­ral parts, which are then assembled into copies of the virus within the host cell. These viral copies are released, leaving the cell either by a process called “budding” (where just one or a few viruses leave the cell at a time) or by a process called lysis (where the cellular membrane ruptures and releases all of the virus particles at once). Both processes usually kill the host cell. The new viruses then infect surrounding cells, continuing the process. Examples of diseases that are viral in origin are influenza (swine flu), some types of pneumonia, poliomyelitis, cold sores and shingles, and AIDS (acquired im­munodeficiency syndrome).

Of course, host cells have several defenses against the viral attacks. For example, in animals (including humans), viral infection leads to the synthe­sis and secretion of proteins called interferons, which “interfere” with viral replication by helping adjacent uninfected cells become resistant to in­fection. Often, this is not enough to stop the spread of infection, and the body’s immune system can cause fever, achiness, tiredness, and other de­fenses, making the person feel “sick” but acting to help the body fight off the attack. Eventually, the virus is completely removed, and the symptoms subside.

HIV (human immunodeficiency virus) is an exception to this situation because HIV infects cells of the immune system that are necessary to kill the infected cells. So, although HIV does not itself directly cause the con­dition known as AIDS, the eventual death of immune cells allows other in­fections to spread (called secondary infections).

So far, no agents have been identified that are secreted by a cell that ac­tually kills a virus. Although antibiotics are effective against bacteria, they do not kill viruses. Recently, there have been agents called antivirals de­signed in the laboratory and isolated from natural sources that are being used to fight certain viral infections. For example, protease inhibitors are used to inhibit the replication of HIV.

References

Crawford, Dorothy H. The Invisible Enemy: A Natural History of Viruses. New York: Oxford University Press, 2000.

Voyles, Bruce A. The Biology of Viruses. New York: McGraw-Hill, 1993.