The arthropods constitute an extremely diverse phylum, in numbers of species and in the range of lifestyles adopted. Species parasitic on plants or animals can be found throughout this phylum. Some groups are entirely parasitic, whereas others have only a few parasitic representatives among a large number of primarily free-living species. Most species are ectoparasites. Some species are directly responsible for morbidity and mortality of their hosts. Many arthropods serve additionally as intermediate hosts for helminth parasites, or vectors for serious protozoan, bacterial, or viral diseases.