Quarantine means Forty Days
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.
The word "quarantine" originates from quarantena, the Venetian language form, meaning "forty days". This is due to the 40-day isolation of ships and people practised as a measure of disease prevention related to the plague.
https://en.wikipedia.org/wiki/Quarantine