I have just started - well about 5 game years ago - a new game in idyllic valley in pacifist mode for a change. My villagers have the same layout I used before, and plenty of wells and no rats (I have a rat catcher in case) but they are dropping like flies from typhoid. The only thing they don’t have is soap but my last game they didn’t get soap for about 40 years and there was only the occasional typoid case.
And why would a child, who lives in a house next door to a well, walks about 5 tiles to school, and has no other reason to be anywhere else, get typhoid? It shouldn’t be totally random!
Check their houses, they might start to accumulate too much shit. So you can add another building (forget its name) to clean it out, or move closer exicting one
Can’t afford another compost house (I have one) as not enough gold coming in yet. I have plopped a soap factory now and that has helped.