The Collaborative International Dictionary of English v.0.48:

coccidium \coccidium\ n.; pl. coccidia. (Microbiol.)
   any sporozoan of the order Coccidia, parasitic on the
   digestive epithelium of vertebrates and higher invertebrates.
   It is the causative agent of coccidiosis, which can be a
   problem in poultry rearing.
   [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

Coccidia
    n 1: an order in the subclass Telosporidia [syn: Coccidia,
         order Coccidia]