A parasite is an organism that feeds and lives on another organism known as a host. Some biologists point out that all animals are parasites because they must rely on other living things for food. Parasites usually live on plants and animals bigger than they are. These organisms only feed on small amounts of the host's tissue or food at a time. They use the food to produce energy, and wastes are released directly into the host's body. The one-celled organism that causes malaria in human beings is a parasite.