Why do people get diarrhea? Normally, the small intestine and colon absorb about 99% of the fluid from what you drink and eat. What remains is stool that is secreted from your GI tract. If there is more water than can be absorbed, or if absorption of water is reduced, it can result in diarrhea. Infections are the most common cause of what is called secretory diarrhea.

Bacteria in the gut invade the lining of the small intestine and produce toxins. The toxins cause secretion of more water than can be absorbed, resulting in diarrhea.
Bacteria can also cause what is known as inflammatory diarrhea.

Absorption in the small intestine is maximized by finger-shaped structures called villi, which project from the wall of the gut. The villi can be damaged by inflammation caused by bacteria, and by your body’s immune response. Destruction of the villi reduces the amount of water absorbed, which results in diarrhea.