Why has there been an explosive worldwide rise in asthma over the past three human generations? Although human genetic variation may predispose some individuals, the dramatic rise in incidence strongly suggests the involvement of environmental changes. The environmental reasoning might be further divided into two alternative explanations. First, our environment has changed because we are "too clean"-in other words, our exposure to soil, dust, farm animals, pets, and "germs" in general is not what it used to be (which is consistent with the "hygiene hypothesis"). Second, the change in our environment concerns our internal milieu, our ancient and conserved microbiome, with continuing changes that result from modernisation.

Read the full text here: the articlehttp://stm.sciencemag.org/content/7/307/307ra152 and editorial.http://stm.sciencemag.org/content/7/307/307fs39.full