Almost all parasites live in or on at least two different kinds of animal during different parts of their lives. Each animal is called a host to the parasite. Worms may live in other mammals, in fish, or in snails. The deer tick lives on meadow mice when it is young, moves to deer to breed, and then waits for another warm-blooded animal on which to feed. Not all parasites need more than one host, however.
Some worms live freely in soil and some protists in water until they can reach a human host.
One of the most complicated lives is lived by the fish tapeworm . Protists Protists are single-celled organisms. Nearly all are microscopic in size. Although protists used to be thought of as one-celled animals or plants, they are often in between these more familiar types. Sometimes animal-like protists are called protozoa.