How do other animal like protists affect other organisms?

Protozoa, although very small, it is difficult to observe with the naked eye, but these animals are directly or indirectly, have a close relationship with humans. Some of the benefit of humanity, and some harmful.

For example: Paramecium can devour bacteria, purify the water; sun worms, insects bell bait fish can be done; dysentery parasite, dysentery who will be within the amoeba dysentery.