Diarrhea can result from various problems, including:

Infection (by parasites, bacteria, or a virus)
Drinking contaminated water or eating contaminated food while traveling in foreign countries.
A variety of infectious organisms can cause "traveler's diarrhea." (See Tips 294 and 295 in chapter
13, The Healthy Traveler)
Food poisoning
Allergic reactions in the gastrointestinal tract
Emotional upset
Overuse of laxatives
Certain medications, including some antibiotics (like tetracycline, cleocin, and ampicillin)
Diverticulitis (inflammation of tiny sacs protruding from the intestines)
Inflammatory bowel disease (primarily ulcerative colitis and Crohn's disease)

With more fluid than usual being flushed out of your body, dehydration is a potential problem, especially with infants and children, who have less fluid to spare than adults. So the first course of action is to drink plenty of clear fluids, like ginger ale, broth, bouillon, herb tea, or just plain water. Even sucking on ice chips helps.

Other steps to control diarrhea include:

Eat little or no solid food for the first few days. Jell-0 is okay; it counts as a clear liquid.)
When diarrhea is waning, follow a B.R.A.T. diet: ripe bananas, rice, applesauce, and toast. These
foods tend to be constipating, and should be the first things you eat after a bout of diarrhea.
Once the diarrhea has subsided, eat small amounts of semisoft foods, like cooked potatoes. Stay
away from protein and dairy products.
Don't eat high-fiber foods like whole-grain bread and bran cereal.
Avoid eating raw fruits and vegetables, fried foods, and sweets, or drinking coffee, all of which are
hard on your digestive tract.
Limit physical activity until bowel activity returns to normal.
Try Kaopectate or other nonprescription remedy containing bismuth. (Follow package directions
to the letter.)

If diarrhea doesn't let up within 48 to 72 hours, or if you notice blood in your stool, contact your doctor for advice.