Computer illustration of the ciliate protozoan Balantidium coli, an intestinal parasite that may cause ulcers (balantidiasis) in the intestinal tract of humans. It is a rare form of dysentery, which is mainly limited to farm workers and those in close association with pigs. The parasites are common and harmless amongst pigs. They form cysts which pass out of the pigs in their faeces. Humans contract the disease through food or drink contaminated by such cysts.