Cystoisospora belli oocysts are not sporulated (ie, infective) when passed in stool. Thus, direct fecal-oral transmission does not occur.

1. Immature oocysts are excreted in stool. Each oocyst usually contains one sporoblast. The sporoblast divides in two and secretes a cyst wall, becoming 2 sporocysts. The sporocysts divide and produce sporozoites.

2. Oocysts are ingested and excyst. Sporozoites are released and invade the epithelial cells.

3-5. Inside the cells, they multiply asexually. After at least one week, male and female gametocytes develop. Fertilization results in oocysts, which are shed in stool.