Some people are born with allergies to certain foods. And as thir lives progress, they will find that they are allergic (or sensitive) to more foods.
What alerts me to a possible parasite/toxin problem is when an adult suddenly develops a strong reaction to certain foods for no apparant reason. We find that this stems from a congested liver that is usually full of toxins from a parasite infection.
It is truly amazing what you will hear when you choose to listen to your body.