Viruses are tiny cellular parasites. They consist simply of a small piece of genetic material, either ribonucleic acid (RNA) or deoxyribonucleic acid (DNA), encapsulated by a protein coat. All viruses seek to invade the nucleus of compatible cells so that they can replicate themselves. The genetic material contained in the virus determines the mechanics of how the infected cell is forced to produce new viral cells. An RNA virus is classified based on the type of genetic material that it carries and how it directs the host cell to replicate.
A virus remains inactive until it enters a host organism's cell. After it is inside, the virus takes control of the host's genetic material, and it uses the cell's natural replication process to make copies of itself. The copies are then released into the organism, where they infect additional cells, enabling the virus to spread quickly throughout the body. The host cells usually are destroyed when the copies are released, although they sometimes remain viable as carrier cells, depending on the virus.
The distinction between a DNA virus and an RNA virus is based on the type of genetic material found in the viral capsule, or virion, before it joins a host cell. After they are inside the host, DNA and RNA viruses hijack the cell differently, depending on which type they are. DNA viruses, such as varicella-zoster, which causes chicken pox, attach to the host's DNA, which is then converted to messenger RNA to begin the replication process. Most RNA viruses, on the other hand, skip the DNA step and immediately direct infected cells to begin replicating viral cells.
Retroviruses, such as human immunodeficiency virus (HIV), are a type of RNA virus that is programmed to change the host cell's DNA to incorporate it. This enables infected cells to function normally until it is time for the cell to naturally replicate, at which point the virus takes over and copies itself. These viruses are especially problematic because they can remain latent for many years, during which time an infected person might not know to seek treatment and might spread the virus to others.
Common RNA viruses include influenza, measles, mumps and West Nile virus. Viruses consist of only a small piece of genetic code and a protein coat, so they are not responsive to medicines that are designed to kill them, such as antibiotics. Vaccines, on the other hand, can often prevent them from replicating and spreading to adjacent cells, especially if they are given before the virus has made too many copies of itself.
Certain retroviruses are strongly correlated with cancer. Leukemia, for example, occurs in a large number of people infected with human T-lymphotropic virus. This observation, combined with the fact that viruses can trigger cells to replicate uncontrollably, has led some researchers to explore the possibility that an RNA virus might cause at least some cancers.