HIV is evolving to become less deadly and less infectious, according to a new study that has found the virus’s ability to cause AIDS is weakening.
The most successful virus wouldn't kill people, wouldn't hurt people, wouldn't even effect our lives. It would just invade, replicate at a sustainable rate, and stick around forever. In fact, since it now relies on it's host to live, it could attack some other foreign invaders for us. Damned white blood cells.
Yes, a good example would be Ebola. The virus hardly affects bats at all. Now that it jumped species it's causing all sorts of havoc in the human body. This is (I'm told) common in germs that have recently crossed the species boundary. In time, new strains would evolve that don't kill the host.