[bostik]

Law of large numbers. The widespread plagues of the past were never 100% lethal, or we wouldn't be here - a sizable fraction of the population survived, and over several rounds of infection/reinfection gained something we'd think of sterilising immunity.

In the meanwhile, the plagues and their knock-on effects may have eliminated approximately HALF of the entire population of their times.[0]

0: https://en.wikipedia.org/wiki/Black_Death ("is estimated to have killed 30 percent to 60 percent of the European population")