Besnoitiosis is "a protozoan disease of the skin, subcutis, blood vessels, mucous membranes, and other tissues" of animals. It "is endemic in tropical and sub-tropical regions with high infection rate but low mortality"; however, it is rare in other regions.

The causative organisms of the cutaneous disease, and the affected animals include:

- "Besnoitia besnoiti" in cattle (swelling of the lymph nodes, subcutaneous swellings, diarrhea, abortion and infertility)

- "Besnoitia bennetti" in horses and donkeys (characterized by a widespread, serious dermatitis)

- "Besnoitia jellisoni" and "B. wallacei" in rodents

- "Besnoitia tarandi" in reindeer and caribou

- "Besnoitia darlingi" in lizards, opossums, and snakes

- "Besnoitia sauriana" in lizards

- Viscerotropic strains of "B.besnoiti" in African antelope

- An unidentified "Besnoitia" species in goats in Iran, New Zealand, and Kenya

They are treated with antiprotozoal agents. Recent papers have also proposed the use of viruses to treat infections caused by protozoa.

Protozoan infections are parasitic diseases caused by organisms formerly classified in the Kingdom Protozoa. They include organisms classified in Amoebozoa, Excavata, and Chromalveolata.

Examples include "Entamoeba histolytica", "Plasmodium" (some of which cause malaria), and "Giardia lamblia". "Trypanosoma brucei", transmitted by the tsetse fly and the cause of African sleeping sickness, is another example.

The species traditionally collectively termed "protozoa" are not closely related to each other, and have only superficial similarities (eukaryotic, unicellular, motile, though with exceptions). The terms "protozoa" (and protist) are usually discouraged in the modern biosciences. However, this terminology is still encountered in medicine. This is partially because of the conservative character of medical classification, and partially due to the necessity of making identifications of organisms based upon appearances and not upon DNA.

Protozoan infections in animals may be caused by organisms in the sub-class Coccidia (disease: Coccidiosis) and species in the genus "Besnoitia" (disease: Besnoitiosis).

Several pathogenic protozoans appear to be capable of sexual processes involving meiosis (or at least a modified form of meiosis). Included among these protozoans are "Plasmodium falciparum" (malaria), "Toxoplasma gondii" (toxoplasmosis), "Leishmania" species (leishmaniases), "Trypanosoma brucei" (African sleeping sickness), "Trypanosoma cruzi" (Chagas disease) and "Giardia intestinalis" (giardiasis).