Retroviruses
A retrovirus is a virus that contains RNA (ribonucleic acid)
that must be converted to DNA in the host cell and then back again into RNA to
be made into proteins. It invades the body's cells, where it produces more such
RNA viruses, including the AIDS virus and many cancer-causing viruses.
Normal viruses contain DNA that is transcribed and
translated in the host. This means that retroviruses must carry with them
reverse transcriptases (host cells don't have reverse
transcriptases) that will copy RNA into DNA.