These alarming increases are not completely understood. However, one popular explanation has been labeled the hygiene hypothesis. This theory stipulates that all humans are born with an allergic predilection. As grow-ing infants encounter new environmental challenges, such as viruses and bacteria, their immune systems shift away from the allergic profile toward one that is capable of protecting them from infectious organisms.
Agricultural environments, with the presence of animal fecal material, have proven to be particularly instru-mental in helping young immune systems mature away from their allergic origins. In less developed countries around the world, there is still extensive exposure to fecal-derived microorganisms at an early age, resulting in a relatively low prevalence of allergic problems. In the United States, Canada, New Zealand, and the countries of western and northern Europe, children have been increasingly raised in urban, nonagricultural settings in which they are raised apart from other children. This lack of exposure to agricultural animals and manure as well as a lower prevalence of viral upper respiratory infections have been associated with an increased prevalence of allergic diseases.
Another important contributor to the rising prevalence of allergies may be nutrition. Countries with high rates of diseases such as asthma and allergic rhinitis have lower blood levels of certain vitamins, including vitamins C and D. Vitamin D may be particularly important, as it is a critical component of a well-functioning immune sys-tem. Finally, pollution of indoor air (particularly cigarette smoke) and outdoor air have been shown to contribute significantly to the development of allergies and asthma.