That’s just environmental adaptation. The only difference is that as humans, our environment isn’t alive. O2, CO2, H20, and Dollar Generals. Viruses, however, are under constant attack from our immune systems in environments that are inherently hostile, and need to replicate and adapt faster. In fact, given the speed at which outside influences can be thrown at the viruses, and the rate at which a virus can achieve biological perfection, unkillable and lethal to every human it encounters, I’d say viruses were the ideal life form, if it weren’t for the pesky “I just killed my environment, and now I’m gonna die” thing.