Balantidiasis life cycle. Diagram showing the life cycle of the parasitic protozoan Balantidium coli, the cause of balantidiasis. This is the largest protozoan to infect humans. It is spread by ingesting food or water contaminated by faecal matter. Some trophozoites invade the wall of the large intestine, where they multiply. The breaking down the intestinal wall causes ulceration, abdominal pain, diarrhoea and vomiting.
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