What can make you allergic to a type of food?

I use to eat eggs all the time growing up. As I got older, and trying diets for example the Atkins diet, I now can no longer eat eggs!! I get soo sick to my stomach. Does anyone know what causes you to "become"allergic to a type of food you use to eat all the time? HELP I miss my Breakfast FOOD!!
Best answer:
Has there been a time when you went without eggs? Because the bacteria that normally break down your eggs may be gone. Remember, as you try different diets, the reduction or elimination of certain foods for even a short while will reduce the amount of healthy bacteria. This happened to me when I stopped taking milk products. One month later, I tried drinking milk and not only did I get a pain like I ate needles, but I also got that other thing, too (the one in which you can't leave home). Try taking acidopholis, which reintroduces a healthy flora of bacteria into your digestive system. I take a minimum of 4 tablets a day and it worked for me. You can get them at Walgreens in a wonderful strawberry chewable form. Try it and see, because it won't hurt and could help a lot.
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