Not really, diseases that don't kill their hosts survive longer. But evolution is a slow and chaotic process, fast moving pandemics like Covid-19 don't always let it kick in much.
I'm at the limit of my knowledge, but as far as I understand it, some of the processes of maturation introduces the possibility for morphing. If two viruses attack the same cell, just as with a sperm and an egg, you get a "baby" virus that is made from both "parents".
I thought that myself.
According to a specialist in tropical diseases who was interviewed on the BBC said that the more aggressive strains of virus incapacitate or kill their hosts therefor slowing down the spread while those with the milder strains tend to infect more people.
In some people the infection is so mild that they don't even realise they have an infection or else they think its just a seasonal cold or flu.
This means they can infect lots more people without even knowing, recovering from a mild strain of virus must offer some immunity from a more aggressive strain.
That was the opinion of the expert on the BBC anyway.
Dull fun fact time, it wasn't Darwin's phrase either. He used natural selection, someone called Herbert Spencer came up with the phrase which Darwin thought was excellent and he started to use it too (whilst giving Credit to Spencer).
It's a very clear summary of the concepts but does confuse some parts of evolutionary theory. This being a good example of it.