How to get Mr. Super-Clean achievement

It's scuffed, bro. You will get random plague outbreaks because there was outbreak in other city or province that you don't even own, or you will get it just because. I finished campaign as Palmyra without getting outbreak in cites I owned and didn't get the achievement.
For some reason, the plague mechanics work more stable when playing Gothi, I didn't get any plague because it was in other city in province, also Gothi gave me the achievement.
Whenever merchant at docks event pop-up, ALWAYS choose action of sending them away.
Make sure to always have 1k for rat-cachers whenever pop-up shows up. It should be next turn after you get notifictation of having rats in city. Don't go for cats, praying or other action. Just don't.
Usually having one religious building (pink one) on level 3 in every city/village is enough. Don't build 4 level building that you don't really need. Check your provinces every few turns.
Even when you have 0% chance for plague outbreak in every city, you can still get outbreak (good job CA). It happened only once during my campaign, so just reload or use autosave and probably it won't happen same turn.
They have red thingie under province screen.
They lower sanitation level so keep eye on it. Destroy buildings that provide squalor. Destroy buildings providing squalor- as many as it takes.

Hope it will help you get the clean boi achievement. Or... you can probably boost it online, having other player leaving campaign and winning on first turn.