Protozoans can either be free-living or parasitic in nature. Meaning they can live in the outside world without a host mostly in water or they can be parasitic in nature and live in humans.
Protozoa range in size from slightly more than 1 to more than 100 μm. They are single-celled organisms and have a true membrane-bound nucleus. They do not have a cell wall.
Protozoans are a class of protists, and so are slime molds and alga. All these guys are unicellular eukaryotes. Protozoan comes from latin and greek to mean FIRST – ANIMAL, thought to be the earliest animals to have existed.
