It was a question Dr. Gideon Lack asked often, when giving lectures to fellow allergists and pediatricians on the topic of food allergies: How many doctors in the room had a patient allergic to peanuts?

Normally, “virtually every doctor would have put up their hand,” Lack said. Peanut allergy is one of the most common food allergies, affecting more than 2% of US children, with a similar prevalence in the United Kingdom, where Lack was practicing.

But at a lecture in Tel Aviv, Israel, about 25 years ago, the audience’s response took Lack by surprise. Only two or three out of about 200 raised their hand.

“I said, ‘Wait, this doesn’t make sense,’ ” he recalled recently. “I was practicing in London, which has a big Jewish community, and I was seeing a high frequency of peanut allergy amongst Jewish children who share similar ancestral background.”

Lack and his colleagues’ investigations into why, which played out over the next 15 years, led to last month’s remarkable finding that the incidence of peanut allergies in the US – after a precipitous rise – appeared to have fallen dramatically.

It turned out that the guidance parents and pediatricians – and Lack himself – had been following, to avoid giving peanuts to babies and young children to try to prevent the development of dangerous allergies, was completely backward.

“By thinking we were protecting them, we were actually causing the problem,” Lack told CNN.

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