This is often mentioned but its true only from statistical or 'strategy' point of view, something viruses on their own simply don't possess. They just multiply via various mechanisms, what happens with their environment is well beyong their capability to care or plan around.
Their mutations are random, but its proverbial throwing gazillion bunch of stuff on the wall and seeing what sticks. Their numbers and short life span give them maybe 100 million? evolution speed advantage compared to humans. Maybe much more. Think how humans would change in say 100,000 generations.
The virus of course does not do this consciously, but simple darwinian logic dictates that the strains that are too deadly will kill its pool of hosts too quickly, and die off together with them.
Their mutations are random, but its proverbial throwing gazillion bunch of stuff on the wall and seeing what sticks. Their numbers and short life span give them maybe 100 million? evolution speed advantage compared to humans. Maybe much more. Think how humans would change in say 100,000 generations.