Because many people they knew didn't wash safely back on the shores.
But by your reckoning it's not many, and given Hitler killed 6 million Jews, which is a lot less than 78 million, he didn't kill that many either.
It's an enormous loss of life, but a tiny number for a worldwide pandemic, especially given current travel technologies that would have made ancient plagues orders of magnitude worse than they were.
In comparison, we're talking about the targeted, deliberate, highly organized extermination of two thirds the entire Jewish race in Europe.