It's interesting you should mention the 1918 flu. That one had two waves, the first one less lethal... Evolution is random, add you say, but that's not quite what happened.

It was in the middle of world war 1. Soldiers were infected, and the ones that became the most ill were sent home to recuperate, or die. Either way they spread the virus better than the ones who weren't as ill, and who stayed in the field.

So the usual evolutionary pressure was upside down here.