What causes diarrhea?

you may experience diarrhea as a result of a number of conditions or circumstances. Potential causes of diarrhea include:

a food intolerance, such as lactose intolerance
a food allergy
an adverse reaction to a medication
a viral infection
a bacterial infection
an intestinal disease
a parasitic infection
gallbladder or stomach surgery
Rotavirus is a common cause of childhood diarrhea. Bacterial infections due to salmonella or E. coli, among others, are also common.

Chronic diarrhea may be a symptom of a more serious condition such as irritable bowel syndrome or inflammatory bowel disease. Frequent and severe diarrhea could be a sign of intestinal disease or functional bowel disorder.