coccidium, (class Conoidasida), any of a large group of protozoan parasites of the sporozoan type. Coccidia live in both vertebrates and invertebrates, primarily in the cells that line the intestine. Infection in humans and other animals results in a disease known as coccidiosis.
The life cycles of coccidia consist of two main phases: a free-living oocyst (encapsulated zygote) phase, in which oocysts are discharged by contaminated animals, and a parasitic sporozoite phase, in which parasites live inside animals. Reproduction occurs in both sexual and asexual phases.
Genera of coccidia include Isospora, Eimeria, Hammondia, Neospora, Sarcocystis, and Toxoplasma.