Asthma, which plagues about 20 million Americans, cuts a curious path across gender lines. During childhood, the disease is more prevalent and more severe among males. After puberty, the opposite is true: Prevalence tilts female, and women face the higher risk of severe cases. For many boys, in fact, asthma symptoms taper off as puberty hits, while for girls that's often when the problems first emerge.

Nobody has determined why this flip-flop exists, but intriguing theories circulate. A study published this week in the American Journal of Respiratory and Critical Care Medicine touches on some of them. One school of thought: Changing levels of the hormones progesterone and estrogen make women more susceptible to attacks, while rising levels of testosterone among boys exert a protective effect. A second: Anatomical differences drive the disparity. Women, for example, are known to develop smaller airways in proportion to a given lung volume than males do. A third: Exposure levels to outdoor environmental triggers have an influence on the shift.

The goal of such scientific sleuthing, of course, is to develop better ways of preventing and treating the disease. While many advances may lie in the future, there's already a take-home message for parents and physicians, according to Kelan Tantisira, the author of the new study and a researcher at Brigham and Women's Hospital. Tantisira says boys who start to feel better after being on asthma medication for years can be more comfortable about getting off their meds.

Click here to read more about changing approaches to treating asthma—and the rising expense of asthma medications.