Allergies are on the rise, experts know. What they don't know is why.
The most popular theory is the so-called hygiene hypothesis. It holds that our culture's addiction to cleanliness, antiseptics and antibiotics prevents our immune systems from developing the ability to ward off real infections. Our bodies then end up overreacting to things they should be ignoring.
In support of this theory are observations that developed countries, which tend to be more squeaky clean, have higher rates of allergies than developing ones, where families are bigger and kids get exposed to more infections.
But not all data fit with the hygiene hypothesis. The theory breaks down in crowded, low-income urban areas in the U.S. Kids there are exposed to more germs. Yet their rates of allergy and asthma remain high.
Other theories include deficiencies in vitamin D, which is already known to have potent effects on parts the immune system, or an excess of dietary folic acid: U.S. rates of allergies and asthma began to rise around the same time that the government mandated fortification of many foods with the vitamin.
However a study published in June in the Journal of Allergy and Clinical Immunology found the opposite -- that people with higher levels of folate in their blood (the natural form of folic acid) were 30% less likely to have allergies and 40% less likely to suffer from wheezing compared with people who have lower (but still normal) folate levels.
It is also possible that altered patterns in the way we feed our kids these days play a role. Recent research suggests that failing to introduce possible allergens early can make allergies more likely to develop later on.