If you are a parent you probably know of a few kids who have life-threatening allergies. And if you are forty-something like me you probably didn’t know anybody with life-threatening food allergies when you were a kid.  It seems like the prevalence of food allergies have increased ten-fold in the last thirty years. Which seems impossible.

Here’s one potential explanation. Suppose that a small percentage of people have a life-threatening allergy to, say, peanuts. And suppose that doctors begin more carefully screening kids for potential food allergies. For example, a kid who gets a rash after eating something is given a skin test or blood test. A positive test correlates with food allergy but does not conclusively demonstrate it. In addition the test cannot distinguish a mild allergy from one that is life threatening.

But life-threatening food allergies are life threatening.  The risk is so great that any child with a non-negligible probability of having it should be restricted from eating peanuts.  Such a child will return to school with a note from the doctor that there should be no peanuts in class because of the risk of a life-threatening allergic reaction.  This is what’s knows as “being allergic to peanuts.”

This is all unassailable behavior on everybody’s part.  And note that what it means is that while there continues to be just a small percentage of people who are deathly allergic to peanuts, there is a much larger percentage of people who, perfectly rightly, avoid peanuts because of the significant chance it could give them a life-threatening allergic reaction.