'Quarantine' comes from the Italian word for 'Forty Days' because during the plague the government started requiring people arriving on ships to stay on the ship for 40 days to make sure no one on board was sick. Why 40 days? Why not 30? Or 70?

by TueuEnSerie
knoperope

You might be interested in this thread while you're waiting:

When there were plagues like the Black Death how did the government dole out medical care? by /u/AlviseFalier

EDIT: And, fun fact, /u/musschrott mentions in this thread that quarantines did use to be thirty days, but were later extended to forty:

What's the history behind the development of quarantine practices worldwide?