For researchers trying to untangle the roots of the current epidemic of asthma, one observation is especially intriguing: Children who grow up on dairy farms are much less likely than the average child to develop the respiratory disease. Now, a European team studying mice has homed in on a possible explanation: Bits of bacteria found in farm dust trigger an inflammatory response in the animals’ lungs that later protects them from asthma. An enzyme involved in this defense is sometimes disabled in people with asthma, suggesting that treatments inspired by this molecule could ward off the condition in people.