Why 'quarantine' literally means 'forty days'.
In 1377 the port city of Ragusa, today's Dubrovnik, ordered ships arriving from plague areas to wait in isolation for 30 days, the 'trentino'. Venice later extended the period to 40 days, 'quaranta giorni' in Italian. That became 'quarantena' and eventually our word quarantine. The number 40 echoed biblical models such as the 40 days of the Flood.
Useful: Take a word apart and you'll often find the practice it once described.
Source: en.wikipedia.org