(covers information from several alternate timelines)
A retrovirus was a biological particle that infected 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. After Taresian DNA was introduced in Harry Kim, The Doctor hypothesised that a retrovirus was most likely responsible. (VOY: "Favorite Son")
In an illusory future set in 2383, Beverly Crusher described William T. 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")