Quarantine is just Venetian for forty days, quaranta giorni. One of the first plague isolation laws, in Ragusa, now Dubrovnik, in 1377, asked for thirty; the forty came later, and nobody seems quite sure why, though Lent and forty days in the wilderness were close to hand. So the length may have been chosen less like a dose than like a fast: long enough to show you meant it. The word has outlived its number. People go into a ten-day quarantine now, or a five-day one, and the forty is still sitting in the middle of the word, keeping count where nobody looks.