Many children in the U.S. have developed severe allergies to certain foods, putting them at risk of death from anaphylactic shock.  There are so many children with nut allergies in the U.S. that many schools do not allow peanut butter in the cafeteria. I am not sure if there has truly been an increase in severe allergies in the past few decades, but I do know that there is no scientific explanation as to why severe food allergies develop. Health practitioners can offer no advice as to how to avoid the development of severe allergies in children. I think this is definitely one of the most important unsolved riddles in biology, especially since it is a life or death problem for so many people.