Dr. Strachan, an epidemiologist, first proposed the “hygiene hypothesis” in 1989. He noted an inverse relationship between family size and the development of atopic disorders and suggested that a lower incidence of infection in early childhood, transmitted through unhygienic contact with older siblings or acquired prenatally, could be the reason for the rise in allergic diseases.
When the theory was developed by experts in allergy and immunology, it broadened into the idea that diminishing microbial exposure is a significant causal element in the current rise in atopy incidence. A wide range of factors, including access to clean water and food, sanitation, vaccinations, antibiotics, birth practices, and unintentional events like the transition from rural to urban life, have been explored as potential causes of changed microbial exposure.
When a disease or collection of diseases spreads quickly without a clear cause, it prompts research to find the root of the problem so that preventive measures can be developed. The increase in the frequency and prevalence of atopic illnesses during the past 30 to 40 years serves as an illustration of this.
Recently, another aspect of Strachan’s hypothesis has drawn significant attention, especially from the media. He claims that the trend towards smaller family sizes as well as “improved household amenities and higher standards of personal cleanliness”—in other words, cleaner homes—is the reason why this key exposure no longer occurs or occurs to an insufficient extent.