means A deadly, fast-spreading infectious disease — especially one that sweeps through a population like a plague.

from From Latin 'pestilentia,' meaning an unhealthy or infected condition, built on 'pestis' — plague, disease, ruin. It traveled into English through Old French 'pestilence' in the medieval period, an age that knew the word's weight all too well. The same 'pestis' root gives us 'pest' — though that little word has softened over centuries from 'a killing plague' to 'an annoying mosquito,' a rare case of language growing less afraid.