Hopital Necker Enfants-Malades, Service d'immunologie, 149 rue de Sevres, Paris, France. jean-francois.bach@nck.ap-hop-paris.fr
According to the hygiene hypothesis, the increased incidence of allergic and autoimmune diseases in developed countries is explained by the decrease in infections. Several questions remain unsettled, concerning notably the nature of protective infections, the mechanisms of protection, the spectrum of diseases concerned by the hypothesis, the difference between triggering and protective infections and lastly the strategies which could be devised to mimic the effect of infections.