How many kinds of viruses are there? Viruses come in all shapes and sizes and have an enormous array of different kinds of organization of basic genetic material within them. And, it is the arrangement and type of genetic material which is the method used for sub-classification of a given group of viruses. For example, the animal virus group can be sub-divided into the following sub-groups: double-stranded DNA; single-stranded DNA; double-stranded RNA; single-stranded RNA, and, retroviruses (a very unique kind of single-stranded RNA virus). An example of a human double-stranded DNA virus is Epstein-Barr virus (EBV). An example of a human single-stranded RNA virus is Influenza virus, Type A). An example of a human retrovirus is Human Immunodeficiency virus (HIV). As you can see, unlike humans, the arrangement and kind of genetic material within viruses can be very diverse. However, for all viruses, regardless of the kind or arrangement of genetic material, the virus is capable of replicating within a living cell and can produce progeny (offspring) which are usually absolutely identical to the original virus. Do viruses change? Sometimes during the process of viral replication, mutations occur. If the mutation is harmful, the new virus particle might no longer be functional (infectious). However, because a given virus can generate many,. many copies of itself, even if 200,000 particles are no good, 100 might still be just fine. Further, some mutations don't lead to harm to the virus, but instead lead to a functional but now brand-new strain of virus (Influenza virus can do this; consequently, there are several different strains of this virus which have to be identified each year in order to make a vaccine against the particular strain which might cause the "flu"). What protects things against viral infection? Humans are protected in a couple of ways. First, if a particular virus infects one or more cells of a given tissue in our body, the infection leads to the synthesis and secretion of substances called interferons. Interferons are proteins and may be designated as alpha, beta, or gamma interferon). These proteins interact with adjacent cells which help adjacent cells become more resistant to infection by the virus. Sometimes, this resistance isn't quite good enough to prevent the spread of the virus to more and more cells, and we begin to feel sick (we are now experiencing a disease caused by the viral infection). Now however, the body's immune system takes over and begins to fight the infection by killing the virus on the outside of the cells, and kills the infected cells, too. The killing of the infected cells prevents the spread of the virus, since as was stated above, a virus requires a living cell in order for the virus to be able to replicate. Eventually, the virus will be completely removed, and we'll get over the illness. HIV is an exception to this situation because HIV infects cells of the immune system which 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 due to infection with HIV allows other infections to harm a person. Recently, there have been agents designed in the laboratory and isolated from natural sources which are being used to fight certain viral infections. These agents are not called antibiotics however, since they are effective only for viruses and have not been isolated from other organisms capable of killing a virus. So far, no agents have been identified which are secreted by a cell which actually kills a virus. You may be familiar with the drug called Acyclovir which is used to inhibit the replication of Herpesvirus; and, AZT and HIV protease inhibitors which are used to inhibit the replication of HIV. Plants are protected from certain viruses by substances which coat leaves and stems and "closing-off" systems which generate a walled-off area within the plant at the source of the infection. Bacteria can be protected from bacterial-specific viruses through the action of enzymes inside the bacterium's cell. However, if a bacterial virus (called a bacteriophage) infects one cell, usually within a very short time, all of the bacterial cells will be killed. If there are no other bacterial cells of that particular species around for that particular virus, however, the virus will die, too. Copyright John C. Brown, July, 2000 - Updated September, 2000.

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