Flectere si nequeo superos, Acheronta movebo.

I do want to specifically shout out Dr Thomas Latta, who is the person who gave us IV hydration, and pretty much magically cured cholera with it in his first attempt. From his diary:

I attempted to restore the blood to its natural state, by injecting copiously into the larger intestines warm water.. trusting that the power of absorption might not be altogether lost, but by these means I produced, in no case, any permanent benefit.. I at length resolved to throw the fluid immediately into the circulation. In this, having no precedent to direct me, I proceeded with much caution. The first subject of experiment was an aged female. She had apparently reached the last moments of her earthly existence, and now nothing could injure her – indeed, so entirely was she reduced, that I feared I should be unable to get my apparatus ready ere she expired. Having inserted a tube into the basilic vein, cautiously – anxiously, I watched the effects; ounce after ounce was injected, but no visible change was produced. Still persevering, I though she began to breathe less laboriously, soon the sharpened features, and sunken eye, and fallen jaw, pale and cold, bearing the manifest impress of death's signet, began to glow with returning animation; the pulse, which had long ceased, returned to the wrist; at first small and quick, by degrees it became more and more distinct ... and in the short space of half and hour, when six pints had been injected, she expressed in a firm voice that she was free from all uneasiness, actually became jocular, and fancied all she needed was a little sleep.

Diarrhea can very easily be death by dehydration, especially when you can't consume oral fluids (Cholera causes extreme vomiting as well). Not only did we solve part of the problem with clean water, the other half was learning how to put clean water into our bodies (with salt).