Protists form a large and varied group of organisms, which fall into three distinct groups based on what they eat. Photosynthetic protists produce energy from sunlight just as plants do, while chemosynthetic protists break down chemicals. The third group, heterotrophic protists, consume outside material as food.
The largest group of heterotrophic protists is the protozoan group. Protozoa eat single-celled organisms such as bacteria and other protists; they do so either by engulfing their prey with their bodies or via animal-like mouth structures. The best-known type of protozoan is the amoeba, which eats by engulfing bacteria.
Protozoa are capable of causing disease in humans. Amoebic dysentery occurs when amoebas infect the human digestive system and feed on the bacteria in the large intestine. Malaria and African sleeping sickness are both protozoan diseases spread by mosquitoes. When an infected mosquito bites a human, the parasites invade the bloodstream.
Slime mold is the other major group of heterotrophic protists. While protozoa directly consume outside material as animals do, slime molds are better compared to fungi in behavior. They absorb nutrients from their environment, rather than ingesting the material itself, and primarily feed upon decaying plant matter. Slime molds are not known to cause human disease.