From the article:

From the constant use of anti-bacterial soap for dish- and hand-washing to air-tight seals around doors and windows, some modern homes have become shrines to hygiene. Add vaccinations and antibiotics to the mix and the body's immune system can get too much artificial reprieve from nature.

"The natural immune system does not have as much to do as it did 50 years ago, because we've increased our efforts to protect our children from dirt and germs," McMorris said in a statement released this month.

Plus, the atrophy of family size means children get exposed to fewer germs than the more kid-filled homes of yesteryear. Families with three or more children, a dynamic that was common decades ago, tend to have fewer allergies as they are exposed to more bacteria- and virus-harboring siblings, McMorris said