Muscles in the intestinal wall push the food through the intestine. Normally, the body absorbs nutrients and water from the food as it passes through the intestine. Diarrhea occurs when the amount of fluid in the stool increases.

The amount of fluid in the stool can increase by three mechanisms:

• The intestine becomes irritated or swollen, which causes water, mucus or blood to leak from the wall of the intestine.
• The muscles push food through the intestine too quickly. The intestine does not have enough time to remove the fluid from the digested food.
• The digested food acts as a sponge, drawing extra water from the intestine, into the stool.

Causes of diarrhea are separated into two groups:

• Infections that cause diarrhea
• Other illnesses that cause diarrhea