Where does 'quarantine' come from?
Venice kept arriving ships waiting for about 40 days during plague times.
The word comes from the Italian 'quaranta', meaning forty, because ships arriving from plague-hit ports in Europe were held offshore for a set period before passengers could land. The CDC's Emerging Infectious Diseases journal traces the practice to Ragusa (modern Dubrovnik), whose Great Council ordered in 1377 that visitors from plague-affected areas be kept apart for 30 days, a period called a 'trentino'.
Over the following decades the practice spread to other cities, and the period was lengthened to 40 days, called a 'quarantino'. Historians are unsure exactly why 40 was chosen. One suggestion is that the longer period seemed to work better; another is that the number echoed the 40-day spans in biblical stories, such as the Flood and the fast in the wilderness. Either way, the choice came long before germ theory, so it was based on observation and tradition rather than a known incubation period.
Quarantine and isolation are different things. Isolation separates people who are known to be sick, which is a far older idea, found in ancient texts on leprosy. Quarantine applies to people who are healthy but may have been exposed. The wording is also still in use: 'quarantine' later came to cover many contexts beyond people, including plants, animals and goods that cross borders.