Asthma is a life-threatening, allergy-driven lung disease common in wealthy countries. But exactly what causes it is unknown. Researchers at Washington University believe the direct cause of asthma is a chemical distress signal produced in skin that is damaged by another hazard of modern life: eczema. Unlike asthma, it is not dangerous, so people rarely worry about it. Nevertheless, 17% of children in America have it, and similarly high figures are found in Australia, Britain, and New Zealand. What is particularly intriguing is that many people with eczema go on to develop asthma (in America, the figure is 70%). That compares with an asthma prevalence of 4-8% in the general population. The Washington University group theorizes that the link between the two conditions is formed by thymic stromal lympho-poietin (TSLP), a signalling molecule secreted by damaged skin cells that elicits a strong immune response from the body to fight off invaders. Thus, eczema-induced TSLP enters the bloodstream and, when it arrives at the lungs, sensitizes them so that they react to allergens that would not previously have bothered them. In other words, they become asthmatic. Several experiments carried out by the researchers, only on mice, have confirmed that skin damage creates susceptibility to asthma by releasing TSLP.