Manufacturers started using soy as a substitute for milk back in the 1950s because they considered it less allergenic than cow's milk [source: Wood]. Ironically, soy now has the notorious distinction of being a member of the big eight -- right alongside milk.
Soy allergies are more common among young children than adults; luckily, most children outgrow the allergy [source: Maleki]. Unfortunately, for those who are allergic to it, soy is everywhere. Its high protein content makes it a common additive in packaged foods as well as health foods.