Asthma is the most common chronic childhood disease. For some reason, rates of asthma keep going up every year. Researchers have been looking at the causes for this increase, which has been found to be much higher in the industrialized world. Everything from exposure to dust mites and cockroaches to diet has been implicated. Now, a new study from the New England Journal of Medicine concludes that there might be another cause: too much cleanliness. The more sterile the early environment for infants six months and younger, the more problems with asthma they seem to have later in life. NPR's Allison Aubrey reports.