Quarantine (from the Italian for "forty days" - a standard quarantine period for plague) describes the use of physical isolation of an environment, usually in order to prevent the spread of a contagious disease. Nowadays, quarantine is rare and techniques such as an isolation room serve much the same purpose. The word is derived from the Italian for "forty days", the period of time ships were generally prevented from landing at port if they were suspected of carrying disease.
Quarantine was historically used to prevent the spread of plague, tuberculosis, leprosy, smallpox and other diseases. The development of the understanding of incubation periods for diseases standardized the timing of quarantine periods in the early 20th century.
The practice was common well into the 20th century, but since the widespread practice of vaccination disease outbreaks have become rare. Nowadays, only anthrax rates quarantine procedures, and then only long enough to ensure decontamination of infected articles like clothing. Animal quarantine is also widely practiced, but even that is being phased out in favor of proof-of-vaccination systems.
On House, there have been two quarantine situations. In Kids, protocols were put in place to keep patients with meningitis from leaving the hospital. In A Pox on Our House, the hospital was put under quarantine because of suspected smallpox.