A leading theory behind the rising allergy and asthma diagnosis rates is the "hygiene hypothesis." This theory suggests that living conditions in much of the world might be too clean and that kids aren't being exposed to germs that train their immune systems to tell the difference between harmless and harmful irritants.
This concept is supported by studies that show that individuals living on farms develop fewer allergic diseases. The theory is that farm animals increase exposure to germs and germ components called endotoxin. These endotoxins stimulate the immune response and decrease allergic inflammation.