The word "quarantine" comes from the Italian phrase quaranta giorni, meaning "forty days." It was the length of time a ship arriving at a Venetian or Ragusan port was forced to lie at anchor before its crew could come ashore. The number is exact, and so is the historical moment: the practice was formalised in the mid-14th century, in the wake of the Black Death.
From Trentino to Quarantino
Before forty days, there were thirty. In 1377, the city-state of Ragusa — modern-day Dubrovnik — passed a law requiring incoming ships and overland travellers from plague-stricken areas to wait thirty days in isolation before entering the city. The Italian word for that period was trentino, from trenta ("thirty"). When the waiting time was extended to forty days, the period was renamed quarantino, and that is the form English borrowed.
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Why Forty?
The choice of forty days was partly medical and partly symbolic. Medically, the period was long enough to outlast the typical incubation and contagious phase of bubonic plague. Symbolically, forty was a number deeply rooted in Christian and Mediterranean culture: forty days of Lent, forty days of rain in the story of Noah, forty days that Christ spent in the wilderness. A "forty-day wait" carried a weight that "thirty-day wait" did not.
A Glimpse of Medieval Public Health
Venice built dedicated quarantine islands — the lazzaretti — in the lagoon. Ships flew yellow flags to signal disease aboard, and goods were aired on racks before being released to the city. The system was not perfect, but it was one of the first state-level public health regimes in Europe and the template for every port quarantine that followed.
Into English
The word entered English in the early 17th century, initially in its literal sense of a forty-day period of isolation imposed on ships. Over time it lost the strict count of forty days and broadened to mean any enforced separation of people, animals, or goods suspected of carrying disease. By the 19th century, it could also be used metaphorically — to "quarantine" an idea or a person was to keep them apart from a community.
A Word That Came Roaring Back
For most of the 20th century, "quarantine" was a quiet, somewhat technical term — familiar to vets, customs officers, and historians of the plague. The COVID-19 pandemic of 2020 changed that overnight. The word was suddenly everywhere, often loosely applied to mean "stay at home." Strictly speaking, quarantine means restricting people who may have been exposed to a disease but are not yet showing symptoms. Isolation applies to those already sick. The distinction is older than either word in English.
Forty in Other Words
The Italian root quaranta ("forty") is itself from Latin quadraginta, and that same Latin root surfaces in English in less obvious places. Quarantain was an old English legal term for the forty days a widow could remain in her late husband's house. Lent, the forty-day Christian fast, is called cuaresma in Spanish and carême in French — both literally "the fortieth."