The Hygiene Hypothesis

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The hygiene hypothesis is also sometimes referred to as the biome depletion theory, or the “old friends” theory, is a hypothesis that states that the cause of allergic disease may be immune intolerance resulting from inadequate exposure to parasites and microbes in childhood.

In other words, the family size, improved household furnishing, increased cleanliness and lower infection rate, along with immunizations against many childhood diseases, have been responsible for atopy. This is because these factors merge to create an environment which offers low chances for cross-infections within a family.


The changes in hygiene standards seen throughout the industrial revolution of the nineteenth and twentieth centuries brought about vast improvements in sanitation, potable water and garbage collection. These are thought to have reduced exposure to many infectious diseases such as cholera and typhoid. However, these measures may also have reduced exposure to helpful microbial substances that help the body to achieve tolerance to potential environmental triggers for allergies.

In 1989, David Strachan suggested that the steep rise in allergic diseases such as asthma and hay fever seen in the 20th century might be attributable to a lower incidence of infection in early childhood. This observation sparked the beginning of the hygiene hypothesis.

Improvements in the standards of household and personal cleanliness were originally believed to be among the most significant factors contributing to this reduced exposrue. Additionally, smaller family sizes were also linked to the idea of reduced microbial exposure, as there was less interpersonal spread of infection that may be needed to maintain immune strength.

Related Conditions

The hygiene hypothesis was originally linked to hypersensitivity  in the form of conditions such as hay fever and asthma. However,  lowered exposure to infectious agents may have a much broader effect than initially believed. In fact, several other conditions have since been associated with the hygiene hypothesis including:

Old Friends Hypothesis

The “old friends” hypothesis was put forward by Graham Rook in 2003 to modify the original hygiene hypothesis, and explain the effect of microbial exposure on the incidence of inflammatory disorders.

This hypothesis is based on the presence of essential microbes that have been present as latent infections, microbes, or carrier states that have been tolerated thoughout human existence. This is in contrast to exposure to other microbes such as those responsible for causing colds, influenza, measles and other childhood illnesses.

It is believed that dependency of the immune system on these microbiotic “old friends” may cause weakened functionality of the immune system.

At present, the “old friends” hypothesis is considered to be the most complete explanation for the effect of sanitation and exposure to microbes on the development of these conditions.

Other Relevant Factors

The research that led the proposal of the hygiene hypothesis was largely observational or epidemiological, and there have since been some animal studies to mimic the effect of sanitation standards.

However, there are several other factors that may be able to account for the increased incidence of allergic disease, independent of the hygiene hypothesis. These include:

  • Dietary changes
  • Environmental pollution
  • Changes in the pattern of physical activity
  • Increasing obesity
  • Stress in modern life
  • Genetic predisposition
  • Changes in the socio-economic status

Further research is required to support the hygiene or “old friends” hypothesis, to link the observations to microbial exposure, without the uncontrolled interplay of these other factors.


Last Updated: Jun 26, 2019

Yolanda Smith

Written by

Yolanda Smith

Yolanda graduated with a Bachelor of Pharmacy at the University of South Australia and has experience working in both Australia and Italy. She is passionate about how medicine, diet and lifestyle affect our health and enjoys helping people understand this. In her spare time she loves to explore the world and learn about new cultures and languages.


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