The Warsaw Ghetto beat an epidemic. Scientists say they know how.

To some witnesses, it simply didn’t make sense.

“This is really an irrational phenomenon,” wrote the Polish historian Emanuel Ringelblum, in notes found after his execution by the Gestapo. “There’s no explaining it rationally.”

It was November 1941 in the Warsaw Ghetto, the sliver of Poland’s capital that the Nazi occupying government had transformed into a squalid holding pen for the area’s Jews, Romani, and others deemed undesirable. Typhus had raged through the community all through the summer and early autumn. But then, just as infection rates were expected to...