This study applies epidemiology directly — tracking how often peanut allergy occurs and which young people are affected. About 14,000 participants are followed from birth, the kind of large group needed to measure how diseases spread through a population. Skin prick tests, double blind placebo controlled food challenges, and medical histories identify genuine allergies. Umbilical cord blood checks whether sensitization was already present at birth. The cord blood showed no signs of peanut allergy at birth, meaning sensitization develops sometime after. The result: roughly 1 in 50 seven-year-olds in the UK reacted to peanuts or tree nuts.