Diarrhoea is not the illness — dehydration is

Diarrhoea is loose, watery stools three or more times in a day. Acute
diarrhoea usually lasts one or two days and then goes on its own; adults in
the United States average one episode a year, young children two.
Chronic diarrhoea — lasting at least four weeks, continually or in bouts
— is a different thing, and is usually a symptom of a chronic disease.

If it clears within a few days, finding the cause is usually unnecessary.
That is worth saying plainly, because the search for a culprit is where most
of the worry goes.

What causes it

• Bacteria from contaminated food or water
• Viruses — flu, norovirus, rotavirus. Rotavirus is the most common cause
  of acute diarrhoea in children
• Parasites from contaminated food or water
• Medicines — antibiotics, cancer drugs, and antacids containing magnesium
• Food intolerances, such as lactose intolerance
• Diseases of the stomach, small intestine or colon, such as Crohn's
• Problems with how the colon functions, such as irritable bowel syndrome
• Stomach surgery, which can move food through more quickly

Sometimes no cause is found. Travellers to developing countries face
traveller's diarrhoea, from contaminated food or water.

When to call someone

The real danger is dehydration — not enough fluid for the body to work —
and it is most serious in children, older adults and people with weakened
immune systems. The thresholds differ, and the difference matters:

Other symptoms that come with it: cramps or abdominal pain, an urgent need
for the bathroom, loss of bowel control. A viral or bacterial cause may add
fever, chills and bloody stools.

Treatment is replacement, not suppression

The treatment is replacing lost fluids and electrolytes. Medicines to
stop the diarrhoea, or to treat an infection, depend on the cause and come
second.

• Adults: water, fruit juices, sports drinks, caffeine-free sodas, salty
  broths. As symptoms improve, soft bland food.
• Children: oral rehydration solutions, which are formulated to
  replace electrolytes as well as water. This is not the same as giving a
  child juice or a sports drink.

Diagnosis, where it is needed, is a physical exam, a review of medicines, a
stool or blood test for bacteria and parasites, and sometimes cutting out
particular foods to see whether it stops. Chronic diarrhoea gets further
tests.

The two kinds you can prevent

Rotavirus has a vaccine, given to babies in two or three doses.
Traveller's diarrhoea is prevented by what you consume: bottled or
purified water for drinking, ice and brushing teeth; tap water boiled or
treated with iodine tablets; cooked food fully cooked and served hot; and no
unwashed or unpeeled raw fruit and vegetables.

*Source: National Institute of Diabetes and Digestive and Kidney Diseases,
via MedlinePlus.*