Collagenous colitis is a type of inflammatory bowel disease that affects the colon. It is a form of microscopic colitis, which causes chronic diarrhea. In all forms of microscopic colitis, the colon appears normal or almost normal on colonoscopy, but the inflammation is visible when a biopsy of the colon is examined under a microscope (hence its name). People with this disease usually have between four and nine watery bowel movements per day, but in rare cases may have more than fifteen. Various other symptoms relating to the gastrointestinal system or other body symptoms may also be present. The cause of Collagenous colitis is not known but it is thought to be multifactorial. This means that both genetic and environmental factors may interact to cause a person to develop the disease. The extent to which genes play a role is unclear, but familial cases have been described. Certain medications, such as NSAIDS and proton pump inhibitors, have been reported to cause or trigger the disease in some people. However, strong evidence directly linking these medications to the disease is still lacking.