Originally, it was a 40-day-long period of imposed isolation, so it's from the Italian for "forty"
... During 
the first wave of cholera outbreaks, the strategies adopted by health... [DOC] [TLE] quarantine facts, information, pictures | Encyclopedia.com articles ... [PAR] The word "quarantine" derives from the Italian quaranta dei (forty days), a 
reminder ... periods and incubation times of the plagues that afflicted medieval 
Europe.