A parasite is an organism that lives on or in a host organism and gets its food from or at the expense of its host. We can see some parasites and cannot see others if they are too small. There are three main classes of parasites that can cause disease in humans: protozoa, helminths, and ectoparasites. Protozoa are microscopic, one-celled organisms that can be free-living or parasitic in nature. They are able to multiply in humans, which contributes to their survival and also permits serious infections to develop from just a single organism. Helminths are large, multicellular organisms that are generally visible to the naked eye in their adult stages. Like protozoa, helminths can be either free-living or parasitic in nature. Tapeworms belong to this group. Ectoparasites are mainly blood-sucking arthropods, such as mosquitos. Malaria is a parasitic infection, and it is spread by mosquitos.