Protists are a taxonomic Kingdom (Protista) of eukaryotes, which include the ciliates, flagellates, amoeba, and algae. It is a very diverse group of organisms with both unicellular and multicellular forms, and are capable of everything from photosynthesis to predatory behavior. The protists are divided into three main groups based on their mode of nutritional acquisition: protozoan (animal-like), algae (plant-like), and slime mold (fungus-like). In many aspects, the most complex eukaryotic cells are found among the single-celled protists having all of the machinery required for complex activities.
Ciliophora are usually completely covered in tiny cilia, which are tiny hairs used for locomotion. These organisms contain a gullet, food vacuole, oral groove, and a contractile vacuoles. They are seen in either rows, spirals, or in tiny tufts.
Zoomastigophora are the flagellates of the kingdom Protista. The only real constant in this group is the presence of a flagella. They are all very diverse. Some are colonial, others are single. Some are unicellular and some are multicellular. This group is also capable of becoming parasitic (i.e. Trichomonas). 
The reproduction process for the kingdom Protista is extremely complex and confusing, but just like their diet, they have three reproductive processes which are isogamy, anisogamy, and oogamy. Broken down into the simplest terms, they are as follows.
Isogamy: The fusing together of two reproductive cells which are both isogametes. This process occurs only in some types of algae and fungus.
Anisogamy: This process uses alternating reproductive cycles in the protist. The first cycle is sexual (using both a male and female organism to reproduce). The second cycle occurs every other generation of protist, and it is in the form of asexual reproduction (only one sex needed to reproduce; in this case the female is all that is needed in the reproductive cycle.
Oogamy: Reproduction using "gametes," which is the process of a small, unmoving female sex cell fusing with a mobile male sex cell.
The kingdom Protista has three basic methods for obtaining nutrition, which are actually classified as types of protists. Protists are mostly heterotrophic, which means relying on others for nutrition. The protozoan is an almost animal-like hunter. The protist finds a single-celled organism and engulfs them with a mouth-like aperture and ingests them, using either phagocytosis or pseudopodia.
The second type of protist is called algal protists. They use photosynthesis to get their food from the sun using carotenoids, pigments, chlorophyll A, and chlorophyll C (on rare occasions chlorophyll B).
The third and final type of protist is called the fungus-like protist. These are very much like mushrooms and other fungi in that they get their nutrition by absorbing it into their bodies from their environment. Their body is a mass of cytoplasm often referred to as a "slime mold". These organisms are often a bright shade of yellow or orange.
- Main Article: Infectious disease
Protozoa can be parasites or predators. In humans, protozoa usually cause disease. Some protozoa, like plankton, live in water environments and serve as food for marine animals, such as some kinds of whales. Protozoa also can be found on land in decaying matter and in soil, but they must have a moist environment to survive. Termites wouldn’t be able to do such a good job of digesting wood without these microorganisms in their guts, presenting a "What came first, the chicken or the egg?"-like challenge for evolutionists.
Malaria is caused by a protozoan parasite. Another protozoan parasite, Toxoplasma gondii, causes toxoplasmosis in humans. This is an especially troublesome infection in pregnant women because of its effects on the fetus, and in people with HIV infection or other immune deficiency disorder.
Amebiasis (also known as Amebic dysentery or Amebic colitis) is a disease caused by a parasitic amoeba called Entamoeba histolytica. The disease is most common in people who live in developing countries that have poor sanitary conditions. In the United States, Amebiasis is most often found in immigrants from developing countries. It also is found in people who have traveled to developing countries and in people who live in institutions that have poor sanitary conditions. Men who have sex with men can become infected and can get sick from the infection, but they often do not have symptoms.
- Karp, Gerald (2008). Cell and Molecular Biology:Concepts and experiments (5th ed.). Hoboken, NJ: John Wiley & Sons. p. 15. ISBN 978-0-470-04217-5.
- Understanding Microbes in Sickness and in Health by the National Institute of Allergy and Infectious Diseases
- Factsheet: Amebiasis by the Division of Parasitic Diseases. Center for Disease Control.