I know the word 'quarantine' in English, and the word 'quarante' (forty) in French. Somehow I had never made the link.
Apparently the term quarantine comes from an old Venetian dialect word 'quaranta', which obviously comes from the same roots.
It all started off with trying to avoid the consequences of the plague (or 'Black Death') in the 14th century. The earliest quarantines were only for 30 days, but later they adopted 40 days as standard.