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This parasite usually lives as a harmless commensal in our large intestine and caecum. It has a worldwide distribution and in addition to humans, it colonizes the large intestine of many wild and domestic mammals, including sheep, dogs, pigs, and monkeys. However, the extent to which zoonotic transfer occurs is uncertain.

The trophozoite is pear‐shaped, 5–15 μm long, and 7–10 μm wide with four free flagellae at the anterior end (Figure 3.7). A fifth flagellum curves back to form an undulating membrane that extends the length of the body and then projects freely from the posterior apex.

Prevalences tend to be higher in children than in adults. Sometimes it is associated with diarrhoea but whether it causes the condition is not known. Similarly, although there is a higher prevalence of P. hominis in patients suffering from gastrointestinal cancer than in healthy patients, whether there is a causative association is uncertain (Zhang et al. 2019).