Infections range from asymptomatic to life threatening, depending on the species and strain of the parasite and the resistance of the host. Structure Protozoa are microscopic unicellular eukaryotes that have a relatively complex internal structure and carry out complex metabolic activities. Some protozoa have structures for propulsion or other types of movement. On the basis of light and electron microscopic morphology, the protozoa are currently classified into six phyla. Most species causing human disease are members of the phyla Sacromastigophora and Apicomplexa.
Cysts are stages with a protective membrane or thickened wall.
Protozoan cysts that must survive outside the host usually have more resistant walls than cysts that form in tissues. Both sexual and asexual reproduction occur in the Apicomplexa. All parasitic protozoa require preformed organic substancesthat is, nutrition is holozoic as in higher animals. The Protozoa are considered to be a subkingdom of the kingdom Protista, although in the classical system they were placed in the kingdom Animalia. The fossil record in the form of shells in sedimentary rocks shows that protozoa were present in the Pre-cambrian era.