Parasitic animal diseases cause severe economic losses, and some may also be transmitted to human beings. They inflict traumatic, lytic, obstructive, intoxicative, allergenic and proliferative damage on the host, or cause loss of nutriment. In addition, the damage done by parasites may enable bacteria and other pathogens to enter the tissues and set up diseases. There are two main varieties of parasites: protozoa and metazoa. The main classes of protozoa are rhizopods, ciliates, flagellates and sporozoans. Metazoa comprise helminths (parasitic worms) of five separate groups: nematoda (roundworms), nematomorpha (gordian worms), platyhelminthes (flukes and tapeworms), acanthocephala (spiny-headed worms) and annelida (leeches); and arthropods, which can be pathogenic 'per se' or may transmit diseases (by virus, bacteria, protozoa and helminths). Main parasitic animal diseases include trypanosomiasis, liver fluke, blood fluke, trichinosis, toxoplasmosis, babesiosis, theileriosis, besnoitiosis, myiasis, leishmaniasis, mange, filariasis. Bacteria, viruses and fungi may also be parasitic.