Newborns lose this opportunity if they are kept in nearly germ-free environments. Later in life, the hypothesis goes, when the child encounters a mundane allergen such as a dust mite, the body's immune system overreacts, closing down airways to combat an invader that otherwise would go unnoticed. This is asthma, Martinez says. His work, published with five colleagues in the New England Journal of Medicine last summer, concluded that children who go to day care or who have older siblings at home are more likely to get infections. These infections may help their immune systems stay in balance and protect against allergic disorders, including asthma.
Some skeptics suggest that the research promotes dirty living conditions.
But Martinez insists he and his colleagues aren't advocating a return to a cavelike lifestyle. I'm not painting a bucolic vision of the way things used to be. But somewhere in the shift from outhouses and unpasteurized milk to anti-bacterial soap and airtight housing, asthma has exploded. Martinez says that if he can figure out what environmental factors have made modern immune systems so susceptible to asthma, people could change their surroundings. Over time, these changes could restore a balance to human immune systems, and reduce or even eliminate people's risk of developing asthma.