Coccidiosis is a highly contagious disease of chickens and other fowls which is widespread and destructive, especially to half-grown chickens. The heaviest losses occur in chicks from 2 to 10 weeks of age. Just prior to and during their first laying season, pullets sometimes become affected with the disease in a chronic form. The cause is a microscopic protozoan parasite, the oocyst form of which is present in the droppings of diseased birds in large numbers. At least three species of the parasite have been recognized in fowls. The severity of the outbreak is thought to be determined by the species of parasite present.
Madsen, D. E., "Circular No. 86 - Coccidiosis of Chicken" (1930). UAES Circulars. Paper 78.