Allergies are an overreaction of the immune system to substances that under normal circumstances would cause no reaction in most people. The results of this immunologic overreaction are symptoms such as rashes, sneezing, watery eyes, and itching.
Asthma is a disease in which the airways become narrowed or even blocked due to inflammation, causing wheezing, coughing, shortness of breath, breathing trouble, and other serious symptoms. In the case of a severe asthma attack, a person may need emergency treatment to restore normal breathing.
In 1989, David Strachan proposed the hygiene hypothesis. He suggested that the rising incidence of allergic disease, such as asthma or allergies, was actually linked to reduced exposure to germs through declining family sizes, more limited exposure to animals, and higher general standards of cleanliness. Strachan reasoned 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 helped our immune systems to properly adapt so they would not overreact to routine environmental stimuli, such as potential allergens.
According to this hypothesis, in the absence of this repeated exposure, our under-stimulated, developing immune system would then be over stimulated by harmless substances like pollen, resulting in the development of allergies.
One example of the hygiene hypothesis comes from comparing the prevalence of allergies in the East and West German populations before and after unification. Before unification, East Germany had more children growing up on farms and in larger families than West Germany; the population also had much lower rates of allergies and asthma than West Germany.
After unification, however, when East Germany developed a more western culture, its rates of allergies and asthma increased to the degree that they now resemble those of West Germany. East Germany also had a quite highly developed healthcare system, so it is less likely that the observed differences in asthma diagnosis were due to different patterns of medical diagnosis rather than true differences in disease prevalence.
Other evidence came from a cohort study done in Denmark that was published in the BMJ. In the study of 13,070 children, researchers found that early and repeated microbial exposure was associated with a decreased risk of developing allergies later in life, while repeated infectious diseases before age 6 months was associated with an increased risk. The risk of atopic dermatitis increased with each infectious disease and decreased with each additional microbial exposure. Microbial exposures included increasing number of siblings, day care, pet ownership, and farm residence. These findings support the hygiene hypothesis as it applies to microbial exposure, but contradict it when an infectious disease develops.
Other studies have also found an association between pet exposure and decreased risk of atopic dermatitis.
However, the results may be less reliable since most studies did not control for the fact that children more likely to have allergies may have avoided having pets.
The original hypothesis has evolved over several years, giving scientists insight to dig a bit deeper into how these factors work together. Today, there may be no clear answers as to why more people have more allergies or asthma, but the focus has shifted toward allergens. Early-age exposure to environments with higher allergen concentrations, such as a farm, playing in a dusty room or a having a pet cat, may be more of a driving force behind developing a tolerance than exposure to bacteria or viruses.
In the end, the hygiene hypothesis is still merely that: a hypothesis. It may eventually teach us a great deal about why allergic diseases occur, or it may (like many other hypotheses) turn out to be simply wrong.