Quarantine

== Signals and flags ==
[[File:ICS Quebec.svg|thumb|The [[International maritime signal flags|signal flag]] "Quebec", also called the "Yellow Jack", is a simple yellow flag that was historically used to signal quarantine (it stands for '''Q'''), but in modern use indicates the opposite, as a signal of a ship free of disease that requests boarding and inspection.]]
Plain yellow, green, and even black flags have been used to symbolise disease in both ships and ports, with the colour yellow having a longer historical precedent, as a colour of marking for houses of infection, previous to its use as a maritime marking colour for disease. The former flag used for the purpose was the "Lima" ('''L''') flag, which is a mixture of yellow and black flags previously used. It is sometimes called the "yellow jack" but this was also a name for [[yellow fever]], which probably derives its common name from the flag, not the colour of the victims (cholera ships also used a yellow flag).<ref>{{cite journal|vauthors=Sehdev PS|title=The origin of quarantine|journal=[[Clinical Infectious Diseases]]|date=1 November 2002|volume=35|issue=9|pages=1071–2|doi=10.1086/344062|pmid=12398064|doi-access=free}}</ref> The plain [[yellow flag (contagion)|yellow flag]] ("Quebec" or '''Q''' in [[international maritime signal flags]]) probably derives its letter symbol for its initial use in ''quarantine'', but this flag in modern times indicates the opposite—a ship that 'requests free [[pratique]]', i.e. that declares itself free of quarantinable disease, and requests boarding and routine port inspection.<ref>[http://flagspot.net/flags/xf~q.html Quarantine Flag history] ''flagspot.net'', accessed 2 February 2020</ref>

Ships in quarantine today would fly either the Q flag alone, meaning "My vessel is 'healthy' and I request free pratique", or the double Q flag (QQ), meaning "I require health clearance".<ref>[https://msi.nga.mil/Publications/ICOS International Code of Signals, page 102.] ''msi.nga.mil'' {{Webarchive|url=https://web.archive.org/web/20200711064159/https://msi.nga.mil/Publications/ICOS |date=11 July 2020 }}</ref>