Strachan first proposed the hygiene hypothesis in 1989 after noticing a connection between a smaller family size or poorer hygiene practices and a higher likelihood of allergies. Prior to 1989, some observational research had been conducted in this area, including a sizable 1958 study of over 17,000 British children that revealed an inverse connection between allergic disorders with the number of elder siblings.

It is thought that the presence of microorganisms helps the human immune system work properly and protects against allergies. The industrial revolution brought about significant improvements in cleanliness standards, which reduced exposure to several bacteria that might normally strengthen the immune system. This was thought to lead to immune system dysfunction.

However, there were some aspects that fail to be explained by the hygiene hypothesis. In 2003, Graham Rook developed the “old friends” hypothesis as a replacement for hygiene hypothesis to explain some of these aspects. Notably, the “old friends” hypothesis places an emphasis on the ancient microbes that were present throughout human evolution, rather than childhood infections that reduced in incidence greatly throughout the same time period.

The rates of allergic rhinitis, also known as hay fever, and atopic sensitization in East German children increased significantly following the reunification of the country, which led to questions about the impact of Western lifestyle on the incidence of hay fever.

The prevalence of asthma has risen by approximately 1% annually on a consistent basis from about 1980, and allergic asthma is thought to cause the majority of this increase, particularly among children. Some recent research appears to show a slowing of the trend of atopic disease as it reaches a plateau.