The Old School of virus development sees drift and shift as the means of evolution. This site shows how the system is also made up of quantum leaps of change to enable systems to regain balance - especially after we humans have upset a local, regional, or global balance. Viruses are nature's way of re-balancing what appears to be out of balance.

No doubt, viruses are also nature's way of spreading genes around. I wonder how much of our own mutations have come about from viruses?