ABSTRACT
Protozoa are the most abundant of all living things. They have flowed into every possible niche. They can be found in all habitats, from tropical tree leaves and oceans to Arctic snows. They are the food of fish, their skeletons form the white cliffs of Dover and other chalk deposits, and they teem in the soil. One of the habitats they have invaded is the bodies of animals. Protozoa can be found in body cavities such as the lumen of the intestine, in blood plasma, in cells such as those of the blood and other tissues, and even in the nuclei of cells. Some of them cause disease, but the better adjusted do not. After all, when an organism kills its host, it must die too.
