Quoting White Chocolate Milk:" Allergies are definitely from the "bubble". Things like ADD and autism I think have exist there was ... [snip!] ... perfect child. There is a greater presence of medical intervention it seems these days so that could also contribute some what."
Allergies can also be genetic and that is why I have sort of held off giving shellfish (shrimp, lobster, crab...) to my toddler since my grandma is allergic.
As for ADD and autism I think there are many parents who just have active kids but don't like it so they write it off as ADHD and drug up their child so they are more complacent.
Also to go along with this... I have read some stories about parents changing/eliminating foods from their kids diets and it was a reaction to the foods that made them act the way they did and not because it was an actual disorder. The one that struck in my head was my grandpa saying there was a student at his school who could not have red40 (or some number of red dye) because it made the child act out basically.