Asthma and allergic diseases: is there a downside to cleanliness and can we exploit it?

When hay fever was first described in the early 19th century it was an uncommon disorder. Since then asthma, allergic rhinitis and eczema, have become common conditions particularly in industrialised western economies. International prevalence studies reveal wide variation in the prevalence of asthma, and allergic disease, but confirm this view although the studies also show these diseases to be by no means rare in most countries. The reasons remain unclear but the "hygiene hypthesis" postulating an inverse relationship between hygiene, in its broadest sense, and allergic disease, fits some of the epidemiology of these diseases and has an associated immunological hypthesis to support it. Recent studies suggest that many hygiene related factors may influence immune development in favour of an allergic phenotype. Antibiotics in the first of life have been associated with increased risks of allergic disease in later childgood, and farming exposures to protection. Interest in hte role of bowel flora in modifying immune development has been suggested as an explanation for the risk associated with antibiotic exposure and has led to studies exploring the effects of modifying bowel flora with probiotics both to influence established allergic disease and to prevent it. The challenge is to continue to reap the enormous benefits that accrued from modifying infectious disease by both public and individual health strategies but at the same time convincing the immune system that it has been exposed to them.