E.coli
a. Septicemia (blood invasion) produces low-grade diarrhea in calves less than 4 days of age. Other signs are arthritis and collapse.
b. Enterotoxaemia E.coli produces profuse watery diarrhea in calves under and up to 4 days of age. The E.coli multiplies in the small intestine and secretes enterotoxin, which increases secretion of water and sodium chloride. This leads to diarrhea, dehydration, weakness and death

Viruses
a. Rotaviruses are the most common cause of diarrhea in calves from 4 to 14 days of age. Antibodies in colostrum provide 48 to 72 hours protection. The virus invades the small intestine villous cells. Once the villous cells are destroyed it takes about 10 days for these cells to regrow. Adults can carry rotaviruses without showing clinical signs.
b. Coronaviruses cause diarrhea in calves from 4 to 30 days of age. These viruses destroy cells in both the small and large intestine. The calves may strain and pass blood or mucus in the feces. Adult cows can also excrete Coronaviruses, especially during times of stress such as calving.

Protozoa
a. Cryptosporidium usually are seen in 1 to 4 week old calves. This parasite invades the small and large intestine causing villous atrophy. Diarrhea is a result of this atrophy leading to malabsorption. Feces are loose to watery or maybe bloody with mucus. Cryptosporidia are resistant to all common antimicrobial and anticoccidial agents
b. Coccidia-see separate handout.