The eukaryotic Protista make up one of the five main groups of living things. It is a mixed group, mainly unicellular, but some are colonial.
Some diseases are caused by parasites, including malaria, cryptosporidiosis, and a variety of diseases caused by worms and flukes which live in our bodies.
Many serious diseases are caused by protists (or protozoans), including amoebic dysentery, African sleeping sickness, malaria, leishmaniasis and Giardia.
Protozoa are single-celled like bacteria, but they have membrane-bound organelles inside their cells, suggesting a very different line of evolution and descent.
Some of the Protists can be cultured and seen under the low power microscope, a few are visible under a hand lens. They are complex single-celled organisms.