Animal VirusesAt one time or another, we have all most likely been infected with a virus. The common cold and chicken pox are two common ailments caused by animal viruses. Animal viruses are intracellular obligate parasites, meaning that they rely on the host animal cell completely for reproduction. They use the host's cellular components to replicate, then leave the host cell to infect other cells.
Viruses gain entry into host cells via several sites such as the skin, gastrointestinal tract, and respiratory tract. Once an infection has occurred, the virus may replicate in host cells at the site of infection or they may also spread to other locations. Animal viruses typically spread throughout the body mainly by way of the bloodstream, but can also be spread via the nervous system. Viruses have several methods to counter host immune system responses. Some viruses, like HIV, destroy immune system cells. Other viruses, such as influenza viruses, experience changes in their genes leading to antigenic drift or antigenic shift. In antigenic drift, viral genes mutate altering virus surface proteins. This results in the development of a new virus strain that may not be recognized by host antibodies. Antibodies connect to specific virus antigens to identify them as 'invaders' that must be destroyed. While antigenic drift happens gradually over time, antigenetic shift occurs rapidly. In antigenetic shift, a new virus subtype is produced through the combination of genes from different viral strains. Antigenetic shifts are associated with pandemics as host populations have no immunity to the new viral strain.