I'm not sure how in my 300+ hours of playing Pokemon I have never found a shiny but somehow I got Pokerus which has a 1/20,000ish chance infection. The Pokemon that has the virus is in my Heartgold version, and I'm starting a new playthrough on my Black version. Heartgold is a Gen. 4 game and Black is a Gen. 5. I'm not even sure if Black has Pokerus but I want to know if the virus could even be transferred between generations. It would speed up my playthrough and help me tremendously. Does anyone know anything about this?
PokeRus is transferable across Gens if the transferred Pokemon is infected with the virus, unfortunately not if the virus is cured.
PokeRus is available in all games starting Generation 2 :)
Source: This and experience