Despite all the attention given to it, milk allergy is not very common. One to three percent of infants will have it, and most will outgrow it by the time they are two or three years old. The symptoms that can be seen with milk allergy include: diarrhea, vomiting, abdominal pains, hives, or a skin rash like eczema, cough and wheezing, and poor growth due to malabsorption. Rarely, a severe reaction called anaphylaxis can occur in which the body goes into shock.
No one is entirely sure why some children have allergies and others don't. There appears to be some genetic component, as food allergies in one family member often indicate that others in the family will have it. Other things such as infections and other illnesses probably play a role in why some children develop an allergic response to a particular food.