a single-celled microscopic animal of a group of phyla of the kingdom Protista, such as an amoeba, flagellate, ciliate, or sporozoan; a protozoan

parasitic protozoa are among the most important pathogens worldwide

a phylum or grouping of phyla which comprises the single-celled microscopic animals, which include amoebas, flagellates, ciliates, sporozoans, and many other forms. They are now usually treated as a number of phyla belonging to the kingdom Protista