Agreed. The history of wars and disasters is quite clear on this. Diarrhea will be a huge, and currently underestimated, killer. A safe water supply, and Clorox and such to make and keep it safe, as well as proper - essentially exquisite - attention to sanitation and waste disposal and such could be life-savers.
Over-the-counter anti-diarrheals such as Imodium for minor cases might help if you can’t get more potent prescription supplies in quantity, but the real deal requires non-oral fluid and electrolyte replacement. Bulk IV fluids will not be practical for most, but enema bags and the ingredients & recipe for a replacement solution can literally “keep your a$$ alive”. It ain’t pretty to think about, but neither is a corpse - especially if it’s yours or your loved ones.
Flies. They are a killer.
I read an account of a British woman who had been a prisoner of war in the Philippines during WWII.
One of the riskiest times she saw was the final week waiting to be rescued by the invading Allied troops. She and a group of others had been taken to an abandoned building in Manila and left there on their own.
While Manila was under attack by the Allies, it wasn't safe to leave the building. They used an open pit toilet inside the building and within the week were beset by illnesses due to flies visiting the toilet first and their food second. If you can't do something about the flies, there will be suffering.
Some of the latrines I used in the military consisted of half of a 55 gallon drum with diesel fuel in it. The barrels were removed daily and burned. You can't have an effective army without controlling the spread of disease.
If you have to have an outhouse, you want it as far away from living quarters and food preparation as possible.