Eimeria is a genus of apicomplexan parasites that includes various species capable of causing the disease coccidiosis in cattle and poultry, among other animals. The most prevalent species of Eimeria that cause coccidiosis in cattle would be E. bovis, E. zuernii, and E. auburnensis. In a young, susceptible calf it is estimated that as few as 50,000 infective oocysts can cause severe disease. The oocysts of what was later called Eimeria steidai were first seen by the pioneering Dutch microscopist Antoni van Leeuwenhoek (1632-1723) in the bile of a rabbit in 1674. The genus is named for the German zoologist Theodor Eimer (1843-1898).