Protists are primitive single-celled organisms, some of which cause human diseases. According to the Centers for Disease Control and Prevention, four basic types of protozoans afflict humans, which are classified in the groups Sarcodina, Mastigophora, Ciliophora and Sporozoa.
One noteworthy protist that affects humans is called Trypanosoma. This protist causes sleeping sickness and Chagas' disease. The organism was discovered in the early 1900s by the Brazilian researcher Carlos Chagas. It is carried and transmitted by the saliva of flies and assassin bugs when they bite humans.
According to the CDC, most protists that afflict humans live in the intestinal tract and are spread via the fecal-oral route. Giardia is a flagellate protist that causes very serious intestinal symptoms for people, and it is often acquired through the ingestion of fouled water.
Other protists are transmitted by insects and other creatures. One particularly damaging protist is called plasmodium, which causes the disease known as malaria. Malaria lives in the bloodstream of humans, and it is spread from one person to another when a mosquito bites an infected human and then bites another human.
The classification of protists is unresolved. Generally, scientists call plant-like protists algae while they call animal-like protists protozoans. Some other protists differ from both of these types, and they resemble fungi.