The ciliates are an extremely large group of protozoans, but the vast majority are free-living or commensal. A few species, however, can cause quite serious diseases.
Parasitic stages are intercellular, living in the intestine or on the body surface. They reproduce asexually by transverse fission or sexually by conjugation and may encyst for transmission between hosts.
The characteristic feature of ciliates is, of course, their covering of cilia which are used for locomotion. They also possess two different sizes of nucleus. Some of the parasitic species have elaborate adaptations for attachment to the host.