Balantidium coli causes balantidiasis. Balantidium exists in two forms: a vegetative trophozoite and a cyst. They are characterized by having two nuclei: a large macronucleus for growth and metabolism and a small micronucleus for cell division.
The cyst form is ingested in contaminated food and water and develops into trophozoites in the large intestine. The large intestines become ulcerated and periods of alternating diarrhea and constipation result.