The element is a clause if there is a subject and a verb, and it is a phrase if both of these parts of speech are not present. What comes after the introductory element is always an independent clause, or complete sentence, on its own, and the introductory element gives meaning to it.
A parasite is an organism that lives on or in a host organism and gets its food from or at the expense of its host. There are three main classes of parasites that can cause disease in humans: protozoa, helminths, and ectoparasites.
This is because a drug will often drive a parasite from one organ of the body to another. There are over 1,000 species of parasites, yet today's medical testing procedures only screen to identify about 50! There are two major categories of parasites: large parasites, which are primarily worms, and small parasites.