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In International maritime signal flags, plain yellow, green, and even black flags have been used to symbolize disease in both ships and ports, with the color yellow having a longer historical precedent, as a color of marking for houses of infection, previous to its use as a maritime marking color for disease.
The present flag used for the purpose is the "Lima" (L) flag, which is a mixture of yellow and black flags previously used. It is sometimes called the "yellow jack", which became a name for yellow fever. Cholera ships also used a yellow flag.
The plain yellow flag ("Quebec" or Q in international maritime signal flags), perhaps derives its letter symbol for its initial use in quarantine, but this flag in modern times indicates the opposite—a ship that declares itself free of quarantinable disease, and requests boarding and inspection by Port State Control to allow the grant of "free pratique".