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.
Image from the Centers for Disease Control and Prevention Image Library.