A microorganism that is smaller than a bacterium that cannot grow or reproduce outside of a living cell. A virus invades living cells and uses their chemical machinery to keep itself alive and to replicate itself. It may reproduce making a perfect copy, or with errors (mutations); this ability to mutate is the reason why some viruses are able to change slightly in each infected person, making treatment difficult. Viruses are the cause of many common infections in humans and are also responsible for a number of rare diseases. Examples of viral illnesses range from the common cold (usually caused by one of the rhinoviruses) to AIDS (caused by HIV). Viruses may contain either DNA or RNA as their genetic material. The herpes simplex virus and the hepatitis B virus are DNA-type-viruses. RNA viruses have an enzyme called reverse transcriptase that allows for the usual sequencing of DNA-to-RNA to be reversed so that the virus can make a DNA version of itself.

RNA viruses include HIV and the hepatitis C virus. Researchers have grouped viruses together into several major families, based on their shape, behaviour, and other characteristics. These include the herpes-viruses, adenoviruses, papovaviruses (including the papillomaviruses), hepadnaviruses, poxviruses, and parvoviruses, counting among the DNA viruses. On the RNA virus side, major families include the picorna-viruses (including the rhinoviruses), calciviruses, paramyxoviruses, orthomyxoviruses, rhabdoviruses, filoviruses, and retroviruses. There are dozens of smaller virus families that fall under these major classifications. Many viruses are host specific, capable of infecting and causing disease in humans or specific animals only.