In 1989, David Strachan proposed that repeated exposure to microbes at an early age, for example as a result of having siblings, owning a pet, living on a farm, or attending day care, actually reduces the likelihood of developing allergic diseases (such as asthma or hay fever ). He reasoned that these early exposures help our immune system to properly adapt, so it would not overreact to routine environmental stimuli, such as potential allergens.

Over the years, some research has backed up Strachan’s hypothesis, and many scientists have come to believe that too hygienic an environment may set the stage for allergic disease later in life.

Since some patients who are exposed to microbes develop infectious diseases while most do not, researchers wondered whether an association existed between the diagnosis of an infectious disease and the subsequent development of allergies. To test this, they compared the prevalence of eczema (a type of allergic disease also known as atopic dermatitis) in children who had been repeatedly diagnosed with infectious diseases, such as colds and otitis media to children who had been repeatedly exposed to environmental microbes without developing these infections. Their study, published in the online version of the May 2004 British Medical Journal found that while early and repeated microbial exposure decreased a child’s risk of developing allergic diseases later in life, repeated infectious diseases actually increased this risk.

About the Study

The researchers enrolled 24,341 pregnant women and their children. Each of the participants was interviewed four times, twice during their pregnancy and twice more when their children were six months and 18 months of age. During the final interview, each mother was asked if her child had either an itchy rash or had been diagnosed with atopic dermatitis within the past two months. They were also asked whether their child had experienced any of the following infectious diseases:

The researchers also collected data on the number of siblings in the family, whether there were pets in the household, whether they live on a farm, and whether the child had been enrolled in day care prior to being six months old.

The Findings

The researchers found that 54% (13,070) of the children had at least one infectious disease during the first six months of life. The majority of these (85%) were colds. They also found that children who had at least one infectious disease prior to six months of age were more likely to develop atopic dermatitis than those who had not been sick. This association increased with each subsequent infection.

The researchers also found that 2638 of the children had developed atopic dermatitis by the time they were 18 months of age. However, unlike with infectious diseases, the prevalence of atopic dermatitis was lower in those children exposed to microbial rich environments, such as living on a farm, having one or more siblings, owning a pet, or attending day care within the first 18 months of life.

How Does This Affect You?

These findings support the hygiene hypothesis as it applies to microbial exposure, but seem to contradict it when an infectious disease develops. They also appear to contradict the results of a study published in the February issue of the Journal of Allergy and Clinical Immunology. In that study, researchers found that not only were children who had fevers early in life less allergy prone, but that the more fevers they had, the more allergy resistant they became. Clearly, more research is needed before we solve the riddle of our increased rates of allergic disease. Until then, while no one is suggesting Americans stop cleaning their houses, if your toddler decides to sample a handful of dirt while playing in the yard, it may not be such a bad thing.