Parasites

A parasite is an organism that depends on another organism, known as a host, for food and shelter. The parasite usually gains all the benefits of this relationship, while the host may suffer from various diseases and discomforts, or show no signs of the infection. The life cycle of a typical parasite usually includes several developmental stages and morphological changes as the parasite lives and moves through the environment and one or more hosts. Parasites that remain on a host's body surface to feed are called ectoparasites, while those that live inside a host's body are called endoparasites. Parasitism is a highly successful biological adaptation. There are more known parasitic species than nonparasitic ones, and parasites affect just about every form of life, including most all animals, plants, and even bacteria.

Helminths are worm-like organisms of which several classes of parasites are found including nematodes (roundworms), cestodes (tapeworms), and trematodes (flukes). Leeches, of the phylum Annelid, are also helminths and considered as ectoparasitic, attaching themselves to the outside skin of their hosts.

Arthropods are organisms characterized by exoskeletons and segmented bodies such as crustaceans, insects and arachnids. They are the most diverse and widely distributed animals on the planet. Many arthropod species serve as carriers of bacterial and viral diseases, as intermediate hosts for protozoan and helminth parasites, and as parasites themselves.