What's the "een" in Halloween?
retrovirus ret·ro·vi·rus (rět'rō-vī'rəs, rět'rə-vī'-)
n. pl. ret·ro·vi·rus·es
A virus of the family Retroviridae.
Any of a group of RNA viruses whose RNA is used as a template inside a host cell for the formation of DNA by means of the enzyme reverse transcriptase. The DNA thus formed is inserted into the host cell's genome. Most retroviruses can cause cancer. Retroviruses also include HIV.