Viruses mutate frequently.
Infection will create some immunity to other similar viruses - up to but probably not not 100%.
If mutation kills the host it will is difficult for the virus to spread to other hosts.
If a virus causes severe illness it will have less opportunity to spread, reducing transmission
Viruses which don't kill the host survive and are able to spread.
Those that spread fastest will dominate the reproductive process.
Therefore, with no planning involved, natural selection will favour those viruses which spread rapidly and whose hosts are relatively unaffected by their existence.
Omicron may be on this pathway!
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