maritimeprofessional.com

v15Bj TIL that the quarantine flag, also called the “Yellow Jack”, is an international signal flag that was last flown during the 2002-03 SARS outbreak. The flag is flown from a ship that is either arriving in port with known serious health problems or that has been placed under quarantine.
p89J4 TIL that the word quarantine comes from an Italian variant of ‘quaranta giorni’, meaning forty days, which was the period that all ships were required to be isolated before passengers and crew could go ashore during the Black Death plague epidemic. They would fly this flag known as “Yellow Jack”.