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.
Lice are parasitic insects that feed on human blood and can infest the head, body, and pubic area. There are three types of lice that infest humans: body louse (Pediculus humanus corporis) head louse (Pediculus humanus capitis) pubic louse (Pthirus pubis)