incase u guys didnt know how plague works, here is the clue:
once u contracted the plague, it will NOT CURE by it self. no matter how long it last, it will not Cure itself.
it will only stop if u put the Character/fleet commander in to a City/port. for 3-4 turns, then it will be automaticly cured. (how ever it means that city will get plagued as well).
and there is a small chance the one who has plague will die, u will see it in faction anouncement as death reason.
"nature cause, caught by plague etc etc"
plague spreads in different ways, move character with plague in to city / ship, will plague the ship commander or the city immediately.
if someone with plague is on the open, and u join up army with him, u will get plagued too. and as i mentioned before, it will not cure itself unless u go in to a city.
how ever, it is possible to have a character with plague in a city that just cured plague. but if u move the character out and go back in again, the city will once again be plagued.