Quarantine - A condition of enforced isolation.
The word comes from a Venetian custom in the Middle Ages when ships arriving from the reported plague-stricken countries were obliged to spend 40 days (Italian "quaranta" or "forty" comes from the Latin "quadraginta," also meaning 40) at the port, in isolation, before being allowed to unload its cargo and crew. Venice, in those days, was the chief European port of entry and Europe had experienced many epiemics of plague. Forty days was supposed to be long enough to kill the infection for goods (and people) by exposure to air and sunlight.
The current usage of the word is not limited to 40 days but "any period" of isolation.
Trivia: Quarantine first appeared as a legal term in 1609, as the period of 40 days in which a widow could remain in her dead husband's house before creditors could seize it.
Sources: answers.com, etymonline.com, Wikipedia