Out of the thousands of species of Protozoa, the majority are free living. However, many species from within the phyla Sarcomastigophora and Ciliophora are parasitic during some part or whole of their life and all members of the phylum Apicomplexa (the sporozoans) are exclusively parasitic.
Many of the sporozoans lead a predominantly intracellular life in their hosts. The protozoans occurring as parasites in human beings and livestock are serious pathogens and cause immense damage to their well being and often high mortality rates among the affected populations. Let us learn about some of the most common and important protozoan parasites of humans.