While they are a "fun" nautical flag today - signifying that a ship is free of disease and requests boarding and inspection - quarantine flags trace their roots from Black Death, the deadly bubonic plague that raced through Europe in the 13th century, killing an estimated 100-200 million people.
The word quarantine is from the Venetian dialect form of the Italian quaranta giorni, meaning "forty days". The quarantine was an early form of disease prevention, isolating those entering a city, often by ship, to prove they were not infected. The newcomers were placed in a restricted location (originally nearby islands) and required to wait out the forty days to see whether symptoms of the plague would develop. Other diseases, including leprosy and yellow fever, lent themselves to the practice of quarantine. Only after the forty days had passed, and no disease was present, were passengers and cargo permitted to be unloaded. Early public health officials thought that forty days was a sufficient period of time for the disease to be identified and either treated or pass through its normal course.
It is thought that yellow fever was the source for the yellow quarantine flag, though other early quarantine flags were green and even black. Prior to their use on ships, quarantine flags were placed on individual houses, to mark them as containing diseased residents, and a warning for others to stay away.
Far from the world of deadly diseases centuries ago, quarantine flags are used by recreational sailors today as a wry way to proclaim "we are healthy". They are manufactured using seaworthy nylon, designed to resist the damaging effects of sun, wind and salt water. Quarantine flags have canvas heading and two large rust-proof brass grommets. They are available 12" x 18", the standard size for all courtesy flags.