What's behind the influx of kids with peanut allergies?

Several years ago I got a letter from daughter #2’s school informing me that a student in her class was highly allergic to peanuts and asked parents not to send their children with peanut products in their lunchboxes. A couple years ago we received a similar note from my youngest daughter’s school. Now my wife (who’s a teacher) tells me that her school just got a new student with a serious peanut and tree nut allergy.
Where did this epidemic come from? I lived on peanut butter sandwiches back in elementary school and would have really been bummed if my mom couldn’t have packed them in my lunch. I’d never even heard of peanut allergies back then.