Protozoa

Protozoa are single-celled eukaryotes that feed on other organisms or organic matter rather than making food by photosynthesis. The term is an informal one, because they do not form a single evolutionary group, and it is often used for amoebae, ciliates, flagellates and the spore-forming parasites.

Amoebae move by extending false feet, ciliates such as Paramecium swim with rows of cilia, and flagellates use one or more flagella. The apicomplexans are parasites with a special complex at the cell tip for invading hosts. Shell-bearing groups such as foraminifera leave a rich fossil record. Antonie van Leeuwenhoek first saw them in the 1670s and called them "animalcules."

Protozoa cause some of the most important human diseases. Plasmodium falciparum and Plasmodium vivax cause malaria, Trypanosoma species cause sleeping sickness and Chagas disease, Giardia and Entamoeba cause intestinal illness, and Toxoplasma infects many warm-blooded animals. Many are carried by insects, so vector control is a main tool against them.

In soil and water, protozoa graze on bacteria and recycle nutrients, forming a link in the microbial food web that supports larger organisms. Their cell biology shows features such as endosymbiosis, since some protozoa host algae or have plastids from secondary endosymbiosis. They are classified within the eukaryotes, alongside algae and fungi.

• Apicomplexa — the parasite group that includes Plasmodium
• Malaria — the most important protozoan disease
• Eukaryote — the domain they belong to