Cryptosporidiosis is a common cause of diarrhea among preweaned dairy calves. In the United States, the most common species of Cryptosporidium found in dairy calves is Cryptosporidium parvum, an important zoonotic species. Cryptosporidiosis is spread by fecal-oral transmission. Calves begin shedding the oocysts as early as 2 days of age, with peak shedding occurring at 14 days of age. Diarrhea generally starts 3 to 4 days after ingestion of the oocysts. Risk factors for the disease include large dairy farms, summer months, feeding of milk replacer, and early feeding of starter grain. Concrete flooring and appropriate cleaning of feeding utensils decreases the risk of disease.