C. diff is a bacterial infection of the large intestine that causes diarrhea, and sometimes colitis (inflammation of the lining of the large intestine). This bacteria is found in the environment. Patients most commonly develop an infection after taking a course of antibiotics, or after being exposed in a hospital or nursing home. This infection is diagnosed by testing the stool. It is treated with a course of antibiotics. Some patients will require multiple courses of antibiotics to clear the infection.