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[Isospora belli]

A	B	C

A, B, C: Oocysts of Isospora belli.  The oocysts are large (25 to 30 µm) and  have a typical ellipsoidal shape.  When excreted, they are immature and contain one sporoblast (A, B).  The oocyst matures after excretion: the single sporoblast divides in two sporoblasts (C), which develop cyst walls, becoming sporocysts, which eventually contain four sporozoites each.  Images contributed by Georgia Division of Public Health.