Most ciliates are free-living forms.
Relatively few are parasitic, and only one species, Balantidium coli , is known to cause human disease. Still others live in great numbers in the digestive tracts of many hoofed mammals , where they serve to stabilize the large populations of symbiotic bacteria that break down cellulose in the animals' food. Some ciliates harbor symbiotic bacteria or algae. Free-living ciliates may be found almost anywhere there is liquid water, but different forms predominate in different habitats. Ciliates in soils tend to be small forms that can form resistant cysts in order to survive long periods of drying.