So you HAD to go and post a thread about it!!! I was honestly about to PM you my short findings so far, but I'll just go ahead and do it here...

As Factionhair pointed out, that file seems to govern this. I edited mine and made it something nuts like 200, but I still get plague almost every turn in at least one settlement. Methinks one needs to start a new campaign to make these changes happen. Am about to do so, will report back with results.