On this link the “mutation” issue is discussed at length:
https://www.youtube.com/channel/UCD2-QVBQi48RRQTD4Jhxu8w
He explains that lots of mutations happen.
If the new form of the disease is too strong it kills everybody quickly, so it can’t spread very far.
If it is too weak, there is no effect on health and so we don’t know about it.
The “Goldilocks zone” (maximum survival rate for virus) for mutations is somewhere in between, but there are probably several variations out there.
long incubation with asymptomatic period (postulated) seems to decrease any benefit to being non-fatal. doesn’t matter if it kills host after a month of spreading, some of it almost silently.