The word “quarantine” comes from the Italian “quaranta,” meaning forty. It refers to the 40-day waiting period that European port cities imposed on arriving ships during the Black Death in the 14th century. But the practice actually started as a 30-day period with a different name, and the story of how it evolved tells you a lot about how humans first learned to fight pandemics.
The First Isolation Law: Ragusa, 1377
The modern concept of quarantine traces back to 1377, when the rector of the seaport of Ragusa (now Dubrovnik, Croatia) issued what was called a “trentina.” That word comes from “trenta,” Italian for thirty. Under this decree, ships arriving from plague-affected regions had to anchor in isolation for 30 days before anyone could come ashore. It was the first known legal framework designed to prevent the spread of infectious disease, and it balanced two concerns that still compete today: protecting public health and keeping trade moving.
In the following decades, other port cities adopted similar rules. At some point, the isolation period was extended from 30 to 40 days. The practice became known as “quarantino,” derived from “quaranta” (forty), and eventually the word settled into English as “quarantine.”
Why Forty Days?
No one knows for certain why officials chose to extend the waiting period to exactly 40 days. Several theories have circulated for centuries, and the real answer may be a blend of all of them.
One explanation points to Hippocratic medicine. Ancient Greek medical theory held that acute diseases ran their course within roughly 40 days, so officials may have reasoned that this timeframe was long enough to reveal whether someone was truly sick. Another theory ties the number to Pythagorean numerology, in which the number four held special symbolic significance. There was also a strong biblical resonance: 40 days was the duration of Jesus’s trial in the desert, the length of Lent, and a recurring number in scripture associated with purification and testing. On a practical level, 40 days was believed to be enough time for “pestilential miasma” to dissipate from bodies and goods through isolation, fumigation, and disinfection. Medieval physicians didn’t understand germs, but they correctly intuited that time and separation reduced contagion.
Venice Built It Into a System
While Ragusa created the legal concept, Venice turned quarantine into a permanent public health infrastructure. In 1423, the Venetian government designated the island of Lazzaretto Vecchio as a facility for isolating and treating people who already had the plague. Then in 1468, a second island, Lazzaretto Nuovo, was dedicated to a different purpose: holding ships, crews, and cargo that had come from plague-affected areas but weren’t yet showing signs of disease.
Lazzaretto Nuovo functioned as a massive screening facility. Historical records describe more than 100 rooms for quarantining sailors along with warehouses where trade goods sat until they were deemed safe. Ships arriving in Venice from infected ports were required to sit at anchor for 40 days before landing. Those that hadn’t yet been inspected flew a yellow flag, a signal that quarantine officers needed to board and evaluate the vessel before it could dock.
What made Venice’s approach remarkable was its permanence. This wasn’t a temporary crisis response. The Lazzaretto islands operated as continuous, government-run monitoring stations for nearly four centuries, from the early 1400s until Napoleon’s conquest of the region in 1797. Venice’s municipal records preserved detailed accounts of the system, making it one of the best-documented public health programs of the pre-modern world.
From Local Practice to International Law
For centuries, quarantine rules varied wildly from port to port. Each city set its own duration, its own procedures, and its own exceptions. That patchwork system held together well enough during plague outbreaks, but it started to buckle in the 19th century when cholera swept repeatedly across continents. The speed and reach of cholera epidemics made clear that isolated local rules couldn’t protect anyone if the next port over had no rules at all.
France proposed an international meeting to standardize quarantine practices in 1834, but progress was slow. It took until 1851 for the First International Sanitary Conference to convene in Paris. Even then, decades of negotiation followed. The real turning point came in 1893, when both the United States and European nations agreed on a fundamental requirement: countries had to formally notify others when communicable diseases appeared within their borders. That agreement laid the groundwork for the international disease surveillance systems that still operate today.
How the Meaning Has Shifted
The original quarantine was specific: hold a ship offshore for 40 days. Today the word has a broader and slightly different technical meaning. In modern public health, quarantine refers to separating and restricting the movement of people who were exposed to a contagious disease but aren’t yet sick. They may have been exposed without knowing it, or they may be carrying the disease without showing symptoms. The goal is to watch and wait, much as those Venetian harbor officials did, to see if illness develops.
This is distinct from isolation, which applies to people who are already sick or have tested positive. Isolation keeps confirmed cases away from healthy people. Quarantine keeps possibly exposed people away from everyone else. The distinction matters because the two groups need different things: isolated patients need treatment, while quarantined individuals primarily need monitoring and time.
The 40-day period itself is long gone. Modern quarantine durations are based on the known incubation period of each specific disease, which can range from days to weeks. But the core logic is identical to what the rector of Ragusa understood in 1377: if you separate people who might be carrying a disease and wait long enough, you can stop an outbreak from spreading into a wider population.