Coccidoisis is a microscopic parasite that lives in your dog's intestines. The most common coccidia found in dogs is called Isospora.

Healthy adult dogs' immune systems are usually strong enough to keep the coccidoisis population in check. In young puppies whose immune systems have not yet fully developed, or in dogs with weakened immune systems due to stress or pre-existing conditions, these parasites can multiply to such a degree that they interfere with intestinal functions.

The primary symptom is persistent and watery diarrhea. In some cases, there will also be some blood in the feces.