Balantidium coli is a parasitic species of ciliate

Ciliate

The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagella but typically shorter and present in much larger numbers with a different undulating pattern than flagella...

 protozoan that causes the disease Balantidiasis

Balantidiasis

Balantidiasis is a protozoan infection caused by infection with Balantidium coli.-Symptoms:Symptoms can be local due to involvement of the intestinal mucosa, or systemic in nature and include either diarrhea or constipation.-Treatment:...

. It is the only member of the ciliate phylum known to be pathogen

Pathogen

A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...

ic to humans.
Balantidium coli has two developmental stages, a trophozoite stage and a cyst stage. In trophozoites, the two nuclei are visible. The macronucleus is long and sausage-shaped, and the spherical micronucleus is nested next to it, often hidden by the macronucleus.