Cryptosporidia are among the unicellular parasites and are highly infectious. The oocysts are the permanent form of the inactive, infectious parasite. They are excreted in the feces of infected animals and are very resistant to the outside world. In the intestine cryptosporida multiply in the intestinal epithelial cells of the brush border. Cryptosporidia cause subclinical infections in adult animals, which heal completely. In calves, severe infection of the enterocytes with Cryptosporidium can lead to malabsorption and consequently weight loss. Particularly affected are calves younger than 4 weeks. Deaths occur only occasionally. However, Cryptosporidia are responsible for secondary infections with rotavirus, coronavirus and E. coli K99 for the calf diarrhea in the first weeks of life.