ABSTRACT
In 1907 Ernest Edward Tyzzer clearly described a protozoan parasite he frequently found in the gastric glands of laboratory mice. Although Tyzzer credited J. Jackson Clarke with the first published description of a parasite resembling Cryptosporidium, neither the dimensions of the organism nor the figures presented by Clarke support Tyzzer's comment. Tyzzer's original description of the life cycle remains remarkably accurate based on confirmation by recent investigators using electron microscopy, except that Tyzzer interpreted the location of the developmental stages of the parasite as extracellular. Transmission studies have been conducted in which oocysts of Cryptosporidium were obtained from an animal of one species and inoculated into animals of other species to determine if the parasite was infective for the recipients. Such studies have yielded valuable information regarding the epidemiology and taxonomy of the genus. Oocysts of Cryptosporidium have been transmitted from one avian species to another.
