Dairy calves are extremely susceptible to gastrointestinal disease during the pre-weaned period. The risk for enteric disease decreases as the calf ages. When a calf is born, it has been exposed to very few if any micro-organisms, and some aspects of its gastrointestinal immune system are not fully developed.

Dairy cows are most susceptible to infectious diseases, such as mastitis and metritis, during early lactation. All animals on a dairy are being exposed to micro-organisms every day; however, most of them do not become clinically diseased.