Quarantine used to mean thirty, not forty
The number in the word changed once, for reasons nobody fully agrees on — and the original number wasn't even the one you'd guess.
When the Black Death hit the Mediterranean port city of Ragusa — modern Dubrovnik — in 1377, the city council passed a law requiring anyone arriving from a plague-stricken area to wait outside the walls for thirty days before entering. They called it a trentino, from the Italian for thirty. It's the earliest formal quarantine law on record.
Within a few decades, cities across the Mediterranean had copied the idea, and the isolation period had crept from thirty days to forty — a quarantino, from quaranta, forty. Nobody kept a clean record of why. Some historians point to Lent's forty days. Others point to the ancient Greek belief that contagious disease reveals itself within forty days of exposure. Both explanations survive; neither is confirmed.
What is confirmed is the direction of the drift: the number went up, not down, as cities learned the hard way that thirty days wasn't a wide enough margin. The word we still use every time a ship, a laptop, or a person gets isolated preserves that failed first guess, fossilized inside the syllables.
We kept the corrected number. We just never bothered to update the story behind it.