Lactose is a sugar found in dairy products. Lactase is an enzyme in the intestine that breaks down lactose into glucose and galactose, which can be absorbed into the bloodstream. Lactose intolerance is caused by the absence of lactase in the intestine. Lactose collects in the intestine, absorbs water, and causes diarrhea.

Lactose intolerance may be caused by:

• An inherited condition
• An underlying illness

Illnesses that may cause lactose intolerance include:

• Celiac disease
• Chemotherapy:
• The administration of medicines that kill cancer cells.
• Crohn's disease
• Diabetes
• Gastroenteritis:
• Rotavirus infection
• Genetic disease
• HIV infection
• Intestinal parasites:
• Giardiasis