An organism that lives in or on and takes its nourishment from another organism. A parasite cannot live independently.
Helminths – Schistosomiasis, another set of very important parasitic diseases, is caused by a helminth (a worm).
Arthropods – The arthropods include insects and arachnids (spiders, etc.), a number of which can act as vectors (carriers) of parasitic diseases.
The term “parasite” came from the Greek “parasitos” (para-, along side of + sitos, food) meaning “eating at the side of, as at the same table.” The sense of the term later changed to that of a poor friend or relative who lived at the expense of another. Not until the 18th century did “parasite” come into English as a biologic term.
The study of parasites is parasitology.
A parasite that lives or feeds on the outer surface of the host's body, such as a louse, tick, or leech, is called an ectoparasite. Ectoparasites do not usually cause disease themselves although they are frequently a vector of disease, as in the case of ticks, which can transmit the organisms that cause such diseases as Rocky Mountain spotted fever and Lyme disease.
A parasite that lives inside the body of its host is called an endoparasite. Endoparasites include organisms such as tapeworms, hookworms, and trypanosomes that live within the host's organs or tissues, as well as organisms such as sporozoans that invade the host's cells