If you believe in evolution there is a theory that bits of our bodies were built up of 'things' that made us stronger. They aren't viruses by definition, just organisms trying to survive, that formed symbiotic relationships with each other and stuck together until they got enclosed within a system. The things that didn't work together leeched off each other until one eventually disappeared and the other evolved to leech off something else, a virus.
So we slowly developed into more complex creatures, and viruses developed in complexity to bypass bettering defenses, making each more lethal to each other so new symbiotic relationships automatically became harder to form.
Ok that's very simplistic, as Myrrdin's links show, lots of things can be good for you under some conditions and bad for you under others...
, was just wondering about this since scientists are always think about those viruses that make you feel sick 
