I wouldn’t say it always works that way. We have highly dangerous (this disease is a comparative joke) viruses that don’t spread so well. They kill the host. They have not seemed to mutate to less dangerous. Even Avian Flu is quite dangerous variant of influenza but how does that work? Is it too much of a strain of a strain? Ebola hasn’t changed its danger quotient, either.
Covid has mutated millions of times.
Covid is a virus that makes billions of copies every day. It is prolific. But in terms of making good copies, the mechanics are bad. A large percent of covid virus reproduction is ineffective and / or the mutations do not “fit the key” of cells.
The key to a mutation is that it is very effective in transmission, but not lethal enough to kill the host. Ebola doesn’t spread because it kills at such a high rate. It never gets a chance to get out of isolation.
Omicron is very infectious, but it won’t kill. That’s why it will end up ripping through the population quickly and will continue to mutate.
Every once in a while, in Nature, a mistake in reproduction, will hit a home run. But it is not often. And when you consider how much they mutate, it’s a one in trillions of cells. It really is hitting the “virus jackpot.”
Unless of course you use CRISPR.