Cryptosporidium, Toxoplasma gondii, Pneumocystis carinii, Acanthamoeba, Giardia lamblia and Naegleria are all examples of protozoa. They are unicellular eukaryotic organisms that exist in all habitable environments on Earth, and many species of protozoa act as parasites on higher animals.
Medical Microbiology explains that more than 50,000 species of protozoa have been classified. The first protozoa to be described was discovered by Anton van Leeuwenhoek between 1674 and 1716 when he observed the parasite Giardia lamblia in his own stool. At some point during the life cycle of a human being, it is almost certain that one or more of the many species of parasitic protozoa make their home in the human body. The consequences of such an infection range from entirely asymptomatic to potentially life threatening. Some diseases caused by protozoa infections include malaria, trypanosomiasis and leishmaniasis. Protozoan-induced diseases can be difficult to eradicate because of the ability of some species to reproduce asexually, and just a few protozoa making their way into the body of a viable host can produce an immense population. For instance, a person infected with malaria could have up to 400 million Plasmodium falciparum per milliliter of blood from an initial exposure of as few as 10 individual organisms. Other species of protozoans include Balantidium coli, Typanosoma evansi, Dientamoeba fragilis and Entamoeba histolytica.