Protozoology is that branch of zoology which is concerned with the group of animals known as the Protozoa. Protozoa are unicellular, heterotrophic eukaryotes that have been studied for more than 300 years, at first as microscopic curiosities, later as organisms causing disease and more recently as important components of ecosystems. Protozoans are common, and they are of particular interest to man because they cause such diseases as malaria, amoebic dysentery, and African trypanosomiasis (sleeping sickness). Certain protozoans known as foraminifera, which have an extensive fossil record, are useful to geologists in locating petroleum deposits. Protozoans also serve as experimental organisms in many studies of cell and molecular biology.