Balantidium coli is a parasitic species of ciliate alveolates that causes the disease balantidiasis.

Balantidiasis is a zoonotic disease and is acquired by humans via the feco-oral route from the normal host, the domestic pig, where it is asymptomatic.

[5] Balantidium coli lives in the cecum and colon of humans, pigs, rats, and other mammals.

B. coli infection in immunocompetent individuals is not unheard of, but it rarely causes serious disease of the gastrointestinal tract.

It can thrive in the gastrointestinal tract as long as there is a balance between the protozoan and the host without causing dysenteric symptoms.

[1][4] Balantidiasis in humans is common in the Philippines, but it can be found anywhere in the world, especially among those that are in close contact with swine.

[6] The disease poses a problem mostly in developing countries, where water sources may be contaminated with swine or human feces.