Like a terrible houseguest, a parasite relies on another organism — one it lives on or in — for its nutrients and gives nothing in return. The parasite benefits, while the host usually suffers in some way. Many parasites, such as tapeworms, live inside the host’s body. But ectoparasites are found outside the host. (“Ecto-“ is from the Greek language, and means “outside.”) Examples include a dog’s fleas and the head lice that spread from kid to kid in the classroom.