Ans. In general free-living ciliates are found in the environment. The parasitic form of ciliates is few in number in the environment. This is the only species found in the human body that causes diseases. Also, there are some parasitic ciliates found in fish that cause disease in fish and may be a matter of concern for aquaculturists. The ones found on invertebrates are commensals and parasites but do not pose any danger to the host or the environment. So barring some, ciliates cannot be termed as harmful.