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):

coccidium
    n 1: parasitic on the digestive epithelium of vertebrates and
         higher invertebrates [syn: coccidium, eimeria]