Allergies and asthma may be closer kin than we knew. More than half the 20 million asthma cases in the U.S. can be attributed to common allergens such as dust mites, according to a study published in the Journal of Allergy and Clinical Immunology. Exposure to some of these — like cat dander — at a young age may offer some protection, researchers suggested, lending support to the idea that we may be getting too clean for our own good. Another study found that children with allergies living in affluent countries, where exposure to allergens is quite low, are almost twice as likely to develop asthma as similar children living in less-developed nations.