I use this video on PBS's YouTube that describes what we know so far, to my knowledge. It is a 4 year old video but I believe it still holds true. He also says that virus' have no fossil record because they're too small and fragile, so it's just because they can't survive that process. They're about as old as life itself, and in that is the question of how did life begin, where did we come from. I'd love to know more myself. Viruses are essentially part of the whole life / death balance if you look at it from a less scientific perspective, and their ability to mutate and infect / spread to survive is what allows us to keep questioning them the way we do.