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.
There are three types of parasites that feast on humans: There are a host of parasitic infections that cause disease in humans. The effect can range from mildly annoying to life-threatening. Malaria is the most prevalent parasitic disease worldwide, killing more than 1 million people each year.