Chatenoud L, Bertuccio P, Turati F, et al. - By performing this matched case-control analysis on incident atopic dermatitis (AD), researchers tested the hygiene hypothesis which suggests decreased exposure to infectious agents in early life as an explanation for the rise of allergic and autoimmune diseases reported over the past decades in high-income countries. This study included 426 outpatient children with a first diagnosis of incident AD (cases) and 426 children visiting a pediatric/dermatological appointment for nonatopic disorders (controls). The controls were matched to cases (1:1). The OR of AD first occurrence was estimated to be 0.35 for children who had suffered ≥ 4 infections vs those with no infections. With increasing number of siblings, experts reported a decreasing trend in risk. An inverse link of pet keeping, especially daily contact with dogs, with AD risk was also revealed. Overall, findings lend support to the hygiene hypothesis in its broad sense. The role of early-life environmental exposures, including pathogens and commensals, as “microbes contact carriers” was suggested, along with their effect on immune system balance early in life.