Necator americanus (Hookworm) and latrines.
Up until the early 1900s, people in the Southern United States would just go #2 by going out to the field and squatting. The problem is that there are parasites, namely hookworm, that can crawl out of old poo and reinfect people by burrowing into their feet when they go out to the same area, to poop again or maybe do some farm work. This is why using "night-soil" as fertilizer is an extremely bad idea. While a few hookworms aren't that bad, too many can cause:
- severe lethargy
- nausea
- loss of appetite
- diarrhea
- abdominal pain
Enter the solution: a pit latrine. Dig a hole deep enough so the hookworms can't find another foot to invade before they die, and problem solved. There's a Radiolab piece on it.
For your point where you want the disease to clear quickly, hookworms have a months-to-years-long residency in the body, but there are other soil-transmitted diseases. Or your hookworm could just have quicker turnover. Also, in your Fire Nation, maybe they could burn/boil/heat (sterilize/pasteurize) their poop if they really wanted to use it as fertilizer. Maybe there's a zoonotic parasite present in other animals' poo that could cause a more transient infection and where there's more of a reason to reuse the manure as fertilizer. Maybe just in an animal that the Fire Nation uses as a beast of burden/livestock?