A retrovirus was a biological particle that infects cells. It reproduced itself by converting its RNA to DNA and incorporating that code into its host cell's genome. The host cell then produced more copies of the retrovirus, possibly leading to its destruction.
Retroviruses could be used as a vector to introduce foreign genes into a cell. The Taresians used retroviruses to transform males from other species into Taresians for mating purposes. (VOY: "Favorite Son")
In an illusory future (set in 2383), Beverly Crusher described Will Riker's memory loss as the result of Altarian encephalitis, a retrovirus. This later proved to be, seemingly, part of the Romulan equivalent of a holoprogram. (TNG: "Future Imperfect")
After a pair of Tarkaleans had been infected with Borg nanoprobes in 2153, Phlox attempted to slow the devices' progress with a modified retrovirus, with no appreciable affect. (ENT: "Regeneration")
In 2375, The Doctor's annual physicals found a nascent alien retrovirus "bouncing" between USS Voyager personnel on decks 10 and 11. He successfully halted its progress. (VOY: "Latent Image")