Originally, it was a 40-day-long period of imposed isolation, so it's from the Italian for "forty"
... L. 
quadraginta, forty] ... An action to isolate another nation, such as a blockade of its 
ports or a ... From the Italian quarantina, 40-a period of days longer than the 
incubation... [DOC] [TLE] Quarantine - definition of quarantine by The Free Dictionary [PAR] An action to isolate another nation, such as a blockade of its ports or a ... [Italian 
quarantena, from Venetian dialectal Italian, quarantine of a ship (so called ... [
C17: from Italian quarantina period of forty days, from quaranta forty, from Latin 
quadrgint] ... 40 days, of detention or isolation imposed upon ships, people, 
animals,... [DOC] [TLE] Lessons from the History of Quarantine, from Plague to Influenza A ... [PAR] Feb 2, 2013 ...