We will not cover worms in this class. Parasites are enormously successful. The global distribution of parasites varies depending on the incidence of appropriate hosts. The majority of protists are free-living in soils and waters.
Some species are parasitic.
Most significant impact on human history of all infectious diseases. Name comes from 17th cent. French surgeon Charles Laveran observed a form of the parasite in fresh blood. Late 1890's, Patrick Manson postulated transmission by mosquitoes. Dr. Ronald Ross observed developing plasmodia in intestine of mosquitoes, established major features of life cycle.