Growing up with fewer early infections may be part of why allergies became so common

In 1989, epidemiologist David Strachan noticed children with more older siblings had lower rates of hay fever, and proposed that reduced early exposure to infections leaves the immune system under-trained and more likely to overreact to harmless things like pollen or peanuts. The idea has since broadened: researchers now think it's less about dirt itself and more about early exposure to a diverse mix of microbes, from siblings to farm animals.

— David P. Strachan, Hay fever, hygiene, and household size — BMJ, 1989