Allergies and asthma are everywhere: Roughly 45 million Americans have allergies, which are exaggerated immune system reactions to benign things like pollen and cat fur. And close to 20 million Americans have asthma, a chronic disease involving the respiratory system (and often the immune system, as well). Both conditions are growing increasingly common, leading experts to think that our environment may be contributing to the problem — and conventional medical treatments aren't doing much to stop it.