Yellow flag (contagion)

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.

Signal flag "Quebec, " 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.

It is sometimes called the "yellow jack", which became a name for yellow fever. Cholera ships also used a yellow flag.[1]

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".[2]

Plain yellow flags are still commonly used to mark a recent death in a neighborhood in cities such as Jakarta, regardless of the cause. They are placed in intersections leading to the home of the recently deceased as direction markers for mourners, and to mark the funeral convoy so that it is given the right of way.[3]