Unlike worms and fleas, coccidia are parasitic single-celled organisms known as protozoa. According to the Merck Veterinary Manual, these protozoa are one of the most common intestinal parasites among dogs. Coccidia can infect a number of animals but are host-specific. This means only a few kinds of coccidia affect dogs — namely Isospora, Hammondia, and Sarcocystis. You may hear your veterinarian describe a coccidia infection using the term coccidiosis. Coccidiosis is a disease that can make animals very sick. You may also hear the word “coccidium” when talking about a singular organism.