The plague has gone on for 50 turns non stop. Once every 3 or 4 turns it announces in green that the plague has been cured but carries on.

I know Its the same original plague because it has diminished down to only killing 1 pop per turn.

I've had it get "cured" only to continue before too. I always assumed someone put it on me more than once. Weird though

it should say how many were successful in your spy history, no?

Yes the history is under main menu/see all news,

It shows my water supply was poisoned 9 times. However I saw the green cure message at least 15 times. It was truly ridiculous, kept going and going and going...eventually it ended around 60 turns after. Maybe some1 who knows teh maths can verify if it sounds normal.