Parasite

Parasite is an organism that lives on or in another organism which is known as host for food and shelter. The best example is the tapeworms, live in the digestive system. The parasite usually gains the benefits such as feeding on host's tissue and getting nutrient while the host my suffer from various diseases and discomforts. 

There are three main classes of parasites that can cause disease in humans which are protozoa, helminths and ectoparasites.