Retroviruses belong to the family of viruses known as Retroviridae. Viruses are infective particles composed of a nucleic acid, a protein coat called a capsid, and sometimes an enzyme. Viruses are classified as DNA (deoxyribonucleic acid) viruses or RNA (ribonucleic acid) viruses depending on their type of nucleic acid. Retroviruses contain RNA, as well as the enzyme, reverse transcriptase, and a protein coat. Different from other RNA viruses, retroviruses replicate as DNA in the host cell. (Other RNA viruses replicate as RNA.) By using reverse transcriptase, the RNA template synthesizes a DNA copy. This procedure reverses the usual flow of information (DNA to RNA), hence the name retrovirus. The copy enters the host's chromosome as a provirus and may remain dormant, replicating along with the chromosome. It can also become active and replicate more retroviruses. Most retroviruses exist as parasites in animal cells.

HIV (human immunodeficiency virus), the...