Flags

[NZL50505]

So a Q flag (yellow) is a quarantine flag requesting clearance into a new country - I used to use this when sailing between UK & Europe many years ago.

But it effectively symbolises that you are ‘disease free’ and requesting entry. Is there another flag to show the opposite ie that you have disease onboard (obviously Covid in this case)?

[Zanshin]

There is no other flag that denotes this (see en.wikipedia.org/wiki/International_maritime_signal_flags), but I'd use the Q-Flag because it also implies that nobody may come aboard and only the Captain may leave the ship and that only to clear in.