Despite the increasing rate of allergic diseases, both in industrialized and in developing countries, the Amish remain exceptionally " and bafflingly " resistant. Only 7 percent of Amish children had a positive response to one or more common allergens in a skin prick test, compared with more than half of the general U.S. population. Even children from other traditional farming families, who still have lower rates of allergic disease than nonfarm children, are more allergic than the Amish. "Certain kinds of farming practices, particularly the very traditional ones, have this extraordinary protective effect in the sense that, in these communities, asthma and allergies are virtually unknown," said Donata Vercelli, a professor of cellular and molecular medicine. "The studies that have been done in these farming populations are critical because they tell us that protection is an attainable goal." During the first year or two of life, a baby's immune system is rapidly developing and highly malleable by environmental stimuli, such as bacteria. Some experts believe that exposing young children to certain types of beneficial bacteria can engage and shape the growing immune system in a way that reduces the risk of allergic diseases later in life. Farm dust contains a hodgepodge of bacteria shed from livestock and animal feed that isn't harmful enough to cause illness, but does effectively train the immune system to become less responsive to allergens later in life.