The Oregon Trail was an educational game often preloaded on many school computers. In it, students tried to get their wagon family all the way to Oregon, following the trail. Almost always, someone died of dysentery, leading to the ever popular meme, "You have died of dysentery."
Dysentery is a disease where the intestine is inflamed, leading to blood in the stool. It can be caused by amoeba or bacteria, even a version of E-Coli. A main treatment of it is to stay hydrated, which can be a challenge if there’s no clean water around. Dysentery has killed everyone from ordinary citizens to King John of England. It has been a longtime killer of soldiers, and for a long time, there wasn’t much soldiers could do. The main treatment is just to stay hydrated, and sometimes in the field of war, there is simply not enough fresh water to supply what is needed.
In the American Civil War, over nine thousand Union men died from dysentery alone, rather than being shot at. It was one of the top three killers of the war. During these times, the connection between hygiene and disease wasn’t known, so doctors didn’t wash their hands or tools before surgery. They didn’t wipe down surgical supplies in between surgeries, either. Cross contamination wasn’t discovered until much later, so many people spread the disease through neglect. Treatment for dysentery was often just drinking a lot of water, or in the Napoleonic Wars, an avid diet that included rice broths.
Today, it is much easier to treat dysentery, and to avoid getting it. Doctors know how to keep their stations clean and drop the risk of infection. The FDA has approved medications to treat it, but diarrhea can still affect soldiers. Overseas, soldiers eat food their stomachs are not accustomed to and the water may not be entirely clean. In the middle of a mission, there is not likely to be a some conveniently located indoor plumbing for them to relieve themselves.
These situations are not often thought of, especially by civilians, but they happen. It is easier now to treat diarrhea, but the disease still affects soldiers in very serious ways. Mary Roach's book on the topic is Grunt: The Curious Science of Humans at War.