descr_disasters.txt:

Code:
event		plague
frequency	20
winter		false
summer		false
min_scale	2
max_scale	5
Now the 20 is probably to be divided by some number that is hidden somewhere and it certainly is higher than 100 as otherwise you'd have massive plague outbreaks every turn. I'm also guessing this is run at end of turn for each settlement a faction owns, simple reason that you see the symbol popup once you press end turn and it only occurs in settlements.