The Cambridge World History of Human Disease

Protozoa are one-celled animals or animal-like eukaryotic, having an organized nucleus, organisms. Older classifications treated the Protozoa as a phylum in the animal kingdom, but modern taxonomists generally consider them members of a distinct kingdom, the Protista, along with other simple eucaryotes. Three phyla, or classes, of Protozoa have species pathogenic for humankind. The first one is sarcomastigophora, the flagellates and amebas, which include trypanosomes, leishmanias, and parasitic amebas. The second one is the Ciliophora, the ciliates, which have only one human pathogen, Balantidium coli, an intestinal parasite with wide distribution but, usually, little clinical significance. The last one is the Apicomplexa, the sporozoans, which include many important pathogens, including the four species of Plasmodium that cause malaria.