An infection of the large intestine by the ciliated protozoan Balantidium coli, causing diarrhea, abdominal pain, and in severe cases ulceration of the colon wall. Pigs are the main reservoir, and humans typically become infected through water or food contaminated with pig feces, making it an occupational risk for pig handlers and a hazard where sanitation is poor. Most infections are mild or asymptomatic, but the organism can invade the intestinal lining in vulnerable individuals. Diagnosis rests on finding the large, distinctive parasites in stool samples, and treatment with antibiotics is effective. Prevention follows the familiar rules: safe water, thorough cooking, handwashing, and separating pig waste from human food and water supplies.