Does diarrhea occur in your small or/and large intestine?

Sites of diarrhea: Both! doctors often distinguish causes of diarrhea as "secretory" or "osmotic", into which categories may fit inflammatory, ischemic, infectious, functional, malabsorption/maldigestion. Any of these problems can be primarily small or large intestinal. A healthy, normally-motile colon can absorb up to a gallon of fluid a day, and a healthy, normally-motile small intestine can absorb twice that.