Infection often occurs in immuno-compromised individuals, notably AIDS patients, and outbreaks have been reported in institutionalized groups in the United States.

[1] Infection causes acute, non-bloody diarrhea with crampy abdominal pain, which can last for weeks and result in malabsorption and weight loss.

[citation needed] People become infected by swallowing the mature parasite; this normally occurs through the ingestion of contaminated food or water.

Infection occurs by ingestion of sporocyst-containing oocysts: the sporocysts excyst in the small intestine and release their sporozoites, which invade the epithelial cells and initiate schizogony.

Upon rupture of the schizonts, the merozoites are released, invade new epithelial cells, and continue the cycle of asexual multiplication.

The oocysts can be visualized on wet mounts by microscopy with bright-field, differential interference contrast (DIC), and epifluorescence.