http://www.marvistavet.com/html/body_coccidia.html
Coccidia are single celled organisms that infect the intestine. They are microscopic parasites detectable on routine fecal tests in the same way that worms are but
coccidia are not worms and they are not visible to the naked eye. Coccidia infection causes a watery diarrhea which is sometimes bloody and can even be a life-threatening problem to an especially young or small pet.