Protozoa ("Protozoa" is plural, not singular) are a group of motile, unicellular microorganisms belonging to the kingdom Protista. "Protozoa" means First (Proto) Animals (zoa). They are believed to have evolved from bacteria and were the first eukaryotes (organisms with nuclei in their cells). They are most common in bodies of water such as puddles, ponds, and oceans and in moist areas such as soil, and plants. Some common protozoa are: Paramecium Amoeba ,Vorticella, Blepharisma ,Stentor, Euglena, Ceratium.
Some of them are parasitic and/or cause illness. An example of this is Malaria which is caused by Plasmodium, a genus of protozoa. Some species of amoebae are parasitic and can cause Amoebic meningoencephalitis. The amoebae eat away at brain cells and cause severe brain damage or, much more likely, death. However, most protozoa are harmless and easily obtainable.