Life cycle of Cystoisospora belli
At the time of excretion, the immature oocyst usually contains one sporoblast (more rarely, two) (1). After excretion, maturation continues, with the sporoblast dividing in two (the typical oocyst now contains two sporoblasts). The sporoblasts secrete a cyst wall, thus becoming sporocysts, and these sporocysts divide twice, producing four sporozoites each (2). 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 (3). Upon rupture of the schizonts, the merozoites are released, invade new epithelial cells, and continue the cycle of asexual multiplication (4). Trophozoites develop into schizonts, which contain multiple merozoites. After a minimum of one week, the sexual stage begins with the development of male and female gametocytes (5). Fertilization results in the development of oocysts that are excreted in the stool (1).
From Centers for Disease Control Parasites and Health website.