Parasites are organisms that exist on or in other living organisms and draw their nourishment from the host and provide no benefit in return. Parasites can be as small as single-celled protozoa or as large as giant intestinal tapeworms whose length can be measured in feet! By and large, parasites are detrimental to the health of the host.
There are two general types of parasites: external parasites (ectoparasites) and internal parasites (endoparasites). External parasites are those found on or within the skin of the host. Internal parasites live within the host, typically in the gastrointestinal tract, liver, heart, and lungs.