Diarrhea is a symptom and its seriousness depends on the cause. An upset tummy from gastric irritation caused by spicy food is mild. Diarrhea caused by typhoid or cholera can be lethal.

The cause of death is fluid loss and electrolyte imbalance. Enough fluid loss will shut down the kidneys. Loss of sodium or potassium can cause heart problems and lead to cardiac arrest. Drinking too much water to combat water loss can dilute electrolytes even further. So there are a lot of pathways, depending on how fast and severe water loss is, and what's done about it.

The trick to preventing this is to not give plain water, but to give the patient water with electrolytes. This is called Oral Rehydration Therapy, and it works almost like magic, reducing mortality by 95%.

ORT solution is just water with some salts and glucose. The salts are sodium and potassium chloride plus sodium citrate. The glucose helps salt absorption because of something called sodium-glucose co-transport in the intestines.

While people knew about salt and fluid loss in diarrhea since the mid nineteenth century, nobody thought of giving them orally. Some doctors used IV fluids for rehydration, but this required a hospital setting.

During WWII, the first real experiments with ORT began, but they were limited in scope and somewhat inconclusive. ORT remained a niche treatment, not practiced by the majority of doctors.

The big change came in 1967, when an Indian doctor published his results in treating a cholera epidemic in Calcutta with a homebrewed ORT solution. He achieved a remarkable survival rate with this treatment, and it generated a lot of interest.

Within a few years, other doctors experimented with their own formulas, and a modified version of the Indian formula was eventually selected, with sodium bicarbonate replaced by sodium citrate. This is standard today.

Since the 1970s, ORT has become the standard treatment for severe diarrhea. It has probably saved more lives around the world than any other single therapy.