The Ragusa quarantine decree

Ragusa, the merchant republic now called Dubrovnik, had been hit by plague several times since 1348. On 27 July 1377 its Great Council tried something new. A decree entered in the city's lawbook, the Liber Viridis, ruled that anyone coming from a place with plague could not enter the city or its territory until they had spent a month in isolation.

The month was called the trentina, after its 30 days. Two small boats watched for incoming ships, whose passengers waited out the month on uninhabited islands; travelers arriving overland were held on the Dance peninsula. Goods were isolated as well as people and left out in the wind and sun, which was believed to clean them.

In 1390, with plague raging in Rome, the government appointed its first plague officers, three nobles who ran the quarantine, controlled entry, assigned the trentina and questioned travelers about outbreaks elsewhere. They directed gravediggers, cleaners, priests and surgeons. Because the authorities had noticed that people who survived plague did not catch it again, women survivors could be taken on as cleaners. In 1397 a Benedictine monastery on the island of Mljet was turned into a quarantine station.

The isolation period was later extended to 40 days, quaranta giorni in Italian, the source of the English word. Nobody knows why 40 was chosen; suggested reasons include Hippocratic ideas about acute illness, Pythagorean number theory and the forty days Jesus spent in the desert. Italian ports and then other European countries adopted the system, and health authorities turned to quarantine again against SARS in 2003.