Diarrhea is defined as and abnormally frequent number of unformed stools. The symptom can vary from mild discomfort and inconvenience to frequent watery stools accompanied by nausea, cramping, vomiting, or fever. Most cases of simple diarrhea are caused by a bacterial infection, particularly from certain pathogenic strains of E.coli. Once these bacteria enter the digestive system, they release a toxin that prevents the bowel from absorbing liquids, causing the feces to be expelled as a watery stool. This type of diarrhea usually begins with a sudden onset of symptoms. A person who suddenly has to run to the toilet in the middle of the night, or accidentally has a "liquid fart," most likely has a bacterial intestinal infection, Sulfur-smelling burps and gas will sometimes be present, though these are also common with diarrhea caused by giardia.

At present, the drug of preference for bacterial diarrhea is naladixic acid (500 mg). An inexpensive, slightly less effective alternative is the combination antibiotic trimethoprim/ sulfamethoxazole, double strength (160 mg/800 mg). Pepto-Bismol has also been found effective for stopping some cases of diarrhea.

Many travelers make the mistake of instantly popping a few Lomotil or loperamide tablets once they have a loose bowel movement. While these two medications are very effective bowel paralyzers and stop diarrhea by constricting the gut, they do not kill the microorganisms causing the problem.

Using antibiotics prophylactically can decrease the likelihood of contracting diarrhea, though there are some drawbacks. Antibiotics can have side effects, such as increased photosensitivity of the skin, which is already a problem in the thin air of Tibet. Extra care must be taken to prevent sunburning, particularly on the back of the hands and the top of the ears. Certain antibiotics can increase the chance of vaginal yeast infections in women.
Some people prefer not to use drugs and attempt to control diarrhea by altering their diet. Eating only simple boiled foods such as plain white rice, boiled noodles, boiled potatoes, plain daal (a popular Nepali lentil dish), and weak black tea seems to help ease the stomach back to normal. Greasy or spicy foods, dairy products (except yogurt), meats, nuts, chocolate, coffee, and alcohol should be avoided. Some people feel that yogurt (zho) can help reestablish the natural bacterial flora in the intestines. Plain yogurt is sold in jars in most of Tibet's city markets and is usually available where nomadic herders are camped.

Dehydration from diarrhea can cause dizziness and weakness. Ample fluid intake should be encouraged, and a packet of rehydration salts added to a quart (liter) of purified water will help restore the body's depleted electrolytic salts.