I had bad diarrhea one year from the flu. I drank plenty of fluids but ended up in hospital anyways since the fluids I was drinking lacked
electrolytes. If you're losing a lot of fluid via vomit, sweat, or diarrhea, etc... then tea, pop (ginger ale), pedialyte, jello, sports drinks, and basically anything without too much caffeine but with sugar/honey and salt are a good idea. Some studies ...