The word quarantine comes from quarantena, the Venetian for ‘forty’. This is because during the times of the Black Death (15th century) ships returning to Venice were ordered to remain on an island for 40 days( which is where we get the word ‘isolation’ coming from the latin ‘insula’ meaning island).
Why 40 days?
The bubonic plague had a 37-day period from infection to death; so 40 days was an ideal length of time to determine the health of a the crew on a returning ship.
What does Isolation mean?
The US health department describes Isolation as:
Isolation is used to separate ill persons who have a communicable disease from those who are healthy.
What does Quarantine mean?
Quarantine is used to separate and restrict the movement of well persons who may have been exposed to a communicable disease to see if they become ill.