Epidemiology of human atopic dermatitis--seven areas of notable progress and seven areas of notable ignorance.

BACKGROUND: This narrative review highlights areas within the epidemiology of human atopic dermatitis (AD) where significant progress has been made and where considerable ignorance still exists. The review is supported by systematic reviews wherever possible, with the purpose of stimulating fresh approaches to human and veterinary research into AD. PROGRESS: Areas of progress include valid and repeatable methods of disease definition, global documentation of disease prevalence and impact, clarification of the role of some genetic factors, such as filaggrin gene mutations, clear evidence that environmental factors are key, as demonstrated by the positive social class gradient and rising prevalence, a possible protective effect of infections in early life, documentation of comorbidities, such as a reduced risk of glioma, and mapping the evidence base through systematic reviews and an online global resource of clinical trials. IGNORANCE: Areas where significant uncertainty still exists include the question of whether AD is more than one disease, the tendency for researchers to look at the same old risk factors, lack of specific environmental risk factors that are amenable to manipulation, inconsistencies in the hygiene hypothesis, sparse knowledge about adult AD, lack of evidence that eczema can be prevented, and little scientific work exploring what causes flares in people with established AD.