A protozoan is a single-celled eukaryotic organism that can either be free-living or parasitic. They are distinguished from other single-celled eukaryotes, such as bacteria and algae, by their complex cell structure. Most protozoa are unicellular, but some species, such as the slime molds, are multicellular.
Protozoa were first defined in 1818 by German zoologist Georg Ehrenberg as animalcules that move about independently. In 1848, Irish naturalist John Lindley expanded this definition to include allsingle-celled organisms with complex cell structures. The term “protozoan” is now used to refer to any member of the Protozoa phylum, which includes over 30 phyla of single-celled eukaryotes.
The majority of protozoa are free-living and can be found in nearly every type of habitat on Earth. Many species are also parasites, living off the nutrients of other organisms. Some protozoa cause disease in humans and other animals; for example, Plasmodium falciparum is the parasite responsible for malaria. Others play important roles in food webs as decomposers or recyclers of nutrients.
The study of protozoa is known as protozoology.