The Hygiene Hypothesis and the Old Friends Hypothesis Explained


What is the Hygiene Hypothesis?

The Hygiene Hypothesis was a theory first put forward in the late 1980s by David P. Strachan a professor of epidemiology. He identified that children from larger households showed less susceptibility to certain diseases and conditions and he theorised that the reason for this was that children with more siblings were exposed to more germs. He also noted that the decreasing number of infections in western countries correlated with an increase in the number of cases of autoimmune and allergic diseases. He concluded that an individual’s susceptibility to disease was increased if they were raised in more sanitised environments.

A study by Dr. Erika von Mutius conducted in the 1990s appeared to back up this theory; She compared data collected on the rates of allergies and asthma in both East and West Germany with the intention to show that children raised in east Germany in dirtier conditions could be more susceptible to allergies. In fact after compiling her research she found the opposite to be the case and that the number of cases of asthma and allergies were far lower in the east. This study led to renewed interest in the hygiene hypothesis around the world.

The Hygiene Hypothesis was the first to suggest that exposure to germs and bacteria is not a bad thing as it helps the child’s immune system develop

As an unfortunate result of our improved understanding of the causes of disease and the spread of disease, we have been conditioned to regard all ‘germs’ or bacteria, viruses and fungi as ‘bad’ and that as responsible parents we need to keep children in clean sanitised environments away from germs.
The Hygiene Hypothesis was the first to suggest that exposure to germs and bacteria is not a bad thing as it helps the child’s immune system develop. In fact, research shows a strong relationship between very over sterilised or sanitised environments and health issues in young children such as allergies and asthma.

The theory behind the Hygiene Hypothesis is fairly straight forward. When Babies are born they come from a sterile environment in the womb and have had no exposure to ‘germs’. As a result their immune system has not yet developed to cope with germs so is highly susceptible to outside influences and infections. It is only after being exposed to different bacteria and viruses that babies immune system is able to develop and strengthen. This has been thought to explain why having a pet at home results in better health and fewer allergies for a baby in later life – as has been shown in various studies.


The Hygiene Hypothesis, however, has been highly contested and undermined by inconsistencies in various studies from around the world. One such counter argument is that the as the number of vaccinations has increased the number of asthma and allergy cases have actually declined.

The “Old Friend” hypothesis was subsequently proposed in 2003 by Graham Rock as a refinement of the hygiene hypothesis. Where the hygiene hypothesis focusses on the importance of germs in developing the strength of an immune system, the old friend hypothesis emphasises the importance of the immune system being exposed to certain microbiota that have been present with us throughout our entire existence. Hence, the name “old friends”. This theory is considered to be the strongest explanation for the existing link between the immune system and our microbiome.

What is the Old Friends Hypothesis?

The Old Friends theorizes that most microbes have existed alongside us throughout our history – dating back to our hunter-gatherer times. These microbes are largely commensal – and have been instrumental in the development of our immune system and in fighting off infection and that we have a symbiotic relationship – we depend on them just as they depend on us.

Likewise many strains and species of bacteria found in the soil, the air, in water and in our food are essential to the development of our immune system. These bacteria have been with us as we’ve evolved helped the immune system grow stronger throughout history and it is these bacteria that children should be exposed to. The theory is that these bacteria have co-evolved alongside mammals throughout our evolution supporting our immune systems

How does the Old Friends Hypothesis differ?

Reflecting back on our history, almost 90% of our evolution is believed to have occurred during our hunter-gatherer stage as well as early farming communities – environments where we were exposed to soil and the decomposing fruits and vegetables. Unlike with the hygiene hypothesis where bacteria or germs as a whole was considered good for immune health, the old friends approach indicated quite clearly that certain types of bacteria some of which are present in mud and water are what is truly necessary.
With the Old Friends Hypothesis, childhood diseases can be explained by a lack of exposure to the right strains of bacteria, leading to an increase in allergies, weight gain and chronic diseases in later life


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