What's the origin of quarantine procedures?

Quarantine, derived from the Latin word quaresma, means forty. So what does 'forty' have to do with restricting the movement of healthy people who may have been exposed to an infectious disease?

Apparently, the practice of quarantine goes back to the 14th century and to the fear of plague epidemics in coastal cities. Ships arriving in Venice from infected ports, in those times, were made to sit at anchor for forty days before landing. This probably started the practice of quarantine and gave it the name.

The story of United States' efforts to prevent the importation of infectious diseases is even more interesting. The first Federal Quarantine Legislation was passed in 1878 due to continued yellow fever epidemics. Based on this law, ships having smallpox, yellow fever, or other contagious diseases aboard were stopped at Bedloe's Island, the Governor's Island and Staten Island; passengers were quarantined before being allowed to enter New York.

In the later part of the 19th century, steamships were used as 'floating hospitals' for quarantine purposes. Incoming ships would first meet these 'floating hospitals' in the lower bay. An officer would visit the incoming ship and inspect it. If any yellow fever or smallpox was found aboard, a signal was sent and steamers belonging to the quarantine service would bear away the passengers to the Swinburne Island, where they were kept till they recovered.

Resources:

Read more about the history of quarantine from the an old issue of Harper's Weekly and from the Centers for Disease Control.