The word we use for restricting the movement of an individual or group of people to prevent the further spread of a disease is often, though mistakenly, used synonymously with the word isolation. Isolation, particularly medical isolation, is the separation of people who are sick with a contagious illness from those who are healthy. Quarantine, by contrast, separates and restricts the movement of people who have been exposed to a contagious illness but do not have symptoms, to see if they in fact become sick.
The word quarantine comes from Venice. For many centuries throughout the Middle Ages and Renaissance, Venice was a renowned seafaring power. The island republic required sailing for everything: what its inhabitants bought, sold, ate, or made came via ship.
The word is a 17th-century Venetian dialect word for the Italian words quaranta giorni which means “forty days.” This was the length of time that those aboard foreign ships were required to be isolated from others before crew and passengers could disembark, especially during the Black Death plague epidemic of the mid-14th century and subsequent flareups. The plague came to Europe from across the steppes of Asia through Constantinople (Istanbul) landing in 1348 at major Italian ports like Genoa, Naples, and Venice. Within three years it has spread to the rest of the European continent. In that short amount of time it killed at least a third of the population, not counting its prior impact on Asia.
Venetian quarantine of ships, signified by the signal flag flown above then in the harbor called the “Lima” or Yellow Jack, proved to be an effective method of halting the spread of the bubonic plague. The earlier practice in 14th-century Croatia of holding a ship for 30 days (trentine) was ineffective because the incubation period for the plague was 37 days from incubation to death.
Bill Petro, your friendly neighborhood historian