Late_Cretaceous

Could retro-viruses play a roll in causing a species to go extinct. Retro-viruses have been demonstrated to play a roll in evolution of species (i.e. the split between chimp and human).

Quetzal

I don't know the answer to your question concerning retroviruses specifically, but it's documented that viral/bacterial/parasite plagues can be implicated in the elimination/extinction of local populations, or in severely restricting ranges. As an example, trypanosomes have been implicated in rendering several million hectares of East Africa cattle-free.