> Like the flu, every year there are new strains (so surviving it doesn’t grant immunity)

Even if it mutates, the immune systems of people who recovered from it this year will still be able to fight it much better next year, because they've learned how to fight that kind of virus. This is why even though the flu mutates every year, it isn't particularly lethal: our bodies have learned how to fight that kind of virus.

If however one were to take the flu to e.g. an uncontacted tribe who'd never encountered influenza before, it'd completely devastate them, much as how viruses from European settlers were estimated to have killed a double-digit percentage of the native American peoples.