The Centers for Disease Control and Prevention's website, CDC.gov, lists parasites that may cause disease in humans. The three main types of parasites are protozoa, helminths and ectoparasites.
Parasitic organisms live in or on a host and depend on the host for food. Protozoa are able to multiply in the host organism and are transmitted through feces or blood. Helminths include worms such as tapeworm and roundworm. Ticks, fleas and lice are classified as ectoparasites.
An example of a parasite is Plasmodium, which is transmitted by mosquitoes via blood. Plasmodium infection causes malaria and kills over half a million people annually.