The Hygiene Hypothesis or Old Friends Hypothesis

The hygiene hypothesis suggests that our bodies are ecosystems that have been damaged by a rapid reduction, in genetic terms, in the number and variety of microorganisms that populate our bodies. It proposes that our immune system, having evolved in circumstances where such exposure to benign, and not so benign, infectious organisms was constant and continuous throughout life, depends on such exposure to develop properly.

The enormous changes in the numbers and variety of microorganisms we are exposed to during our lifetimes has been caused by the wide spread adoption of modern hygienic practices, like sterile drinking water, refrigeration of food, antibiotics, vaccines, modern housing, clothing, etc., in the developed world. That alteration of our "personal ecosystems" has been so rapid in genetic terms that our immune systems have not had time to adapt. The consequence is the rise of autoimmune diseases and immunological disorders.

This neatly explains why autoimmunity and immunological disorders are far, far more common in the developed world than in the undeveloped.

It also suggests that perhaps we might treat immunological disorders by restoring our personal ecosystems. Probiotics and promotion of eating fermented foods like live yoghurt are an example of this. Helminthic therapy, a form of immunotherapy if used to treat immunological diseases, is also based on the hygiene hypothesis. The use of a benign infectious organism, in this case helminths, to treat or prevent diseases involving immune dysregulation, autoimmunity or chronic inflammation.

But which organisms are the most important? A remarkable study was published in 2009 demonstrates for the first time that this central idea contained in the hygiene hypothesis or old friends hypothesis, is correct. We did coevolve with microorganisms and our immune system has been shaped by them. It suggests that the most important class of organisms, in terms of their impact on us, are helminths. That they in particular have driven evolution of the human immune system. In fact, this paper suggests that we have been shaped in fundamental ways by coevolution with helminths.

When one learns that we would die if all bacteria, moulds, etc. were eliminated from our intestines, or that sixty percent of our stool is bacteria by weight, this idea becomes more obvious. We are walking bioreactors, fermentation vessles, and this is clearly not an accident. We depend on them almost as much as they depend on us. We even have an organ devoted to managing our relationship with intestinal microorganisms. One that until recently science suggested was worthless, apparently because scientists did not understand its function at all. Imagine that, an organ of the body that scientists did not at all understand until the 21st Century. Recent research into the function of the appendix suggests that it exists to regulate our intestinal flora and fauna. If we have an organ devoted to managing intestinal microorganisms perhaps we should not be so cavalier in wiping them out with antibiotics on a regular basis?

Viewed through the lens of the hygiene hypothesis the modern obsession or preoccupation with sterility, equating it with cleanliness and goodness, is revealed for what it is. An unhealthy cultural artefact that arose as a consequence of the more immediate and apparent benefits from adopting modern sanitary practices and technologies. Eliminating typhoid and cholera has saved millions of lives in the aggregate since sewers and clean drinking water were introduced in North American and Western Europe for instance. But in so doing we contributed to the rise of the modern diseases involving immune dysregulation, like Multiple Sclerosis, Crohn's, Ulcerative Colitis, Graves Disease, Hashimoto's Thyroiditis, Type I Diabetes, Asthma, Allergy, Coeliac Disease, and Sjogren's Syndrome.

The idea that so radically altering our environments in this way should have unexpected and unwelcome consequences is perhaps more acceptable an idea now what we know how mankind has damaged the global ecosystem, and as we become more aware of the unexpected results of doing so. In the same way the damage we have done to the environments or ecosystems described by our bodies, our personal ecosystems, has produced unexpected and unwelcome consequences. The result, it can be argued, is that tens, or perhaps hundreds, of millions of people are being killed or made very sick by environmental damage right now.

While the results on a global scale are deforestation, extinction of species, global warming, acid rain, etc., on an individual level they are Multiple Sclerosis, Crohn's, Ulcerative Colitis, Graves Disease, Hashimoto's Thyroiditis, Type I Diabetes, Asthma, Allergy, Coeliac Disease, Sjogren's Syndrome, etc. I you suffer from one of these, or almost countless other diseases, you are a victim of mankind's misguided obsession with cleanliness and the damage it has done to the ecosystem defined by your body.

While no one wants to go back to elevated levels of infant mortality and reduced life spans such as prevailed before modern sanitation was introduced, a more nuanced approach does seem necessary.

Perhaps we can accomplish what used to be handled naturally. By deliberately infecting ourselves regularly with benign, infectious organisms (probiotics and live foods already accomplishes this on a small scale) so that we gain the benefits of exposure to these organisms, without any morbidity or pathology. We can have our cake and eat it, too.


Even a casual reader of newspapers and of web sites will have noticed a massive increase in the reporting of allergies and of immune and autoimmune diseases like peanut allergies, Crohn's Disease, asthma, multiple sclerosis and autism, over the last ten or twenty years. But has there really been an increase in the incidence of these conditions or just in the reporting of them?

Rates of these conditions did increase rapidly, it wasn't just the news media blowing things out of proportion. But, epidemiologists suspected initially that heightened public awareness and improved diagnostic testing were the causes of the increase, not an actual rise.

Research has conclusively proven otherwise. Multiple studies have confirmed that seasonal rhinitis (hay fever) and allergic rashes, as well as diseases like Crohn's and MS, are far more common today than previously, particularly in the industrialised world. Nonindustrialised countries and rural areas have lower rates, in some undeveloped areas allergies and these other "modern"are still unknown.

In stark contrast to those undeveloped regions where seasonal allergies are unknown research has shown that in the USA seasonal allergies increased by 31% from 1985 to 1995. It shows that childhood peanut allergies doubled from 1997 to 2002, and that asthma has increased from 35 cases per 1000 in 1982 to 56 cases per 1000 in 1994.

Up to half of the people in developed countries in certain areas suffer from some form of allergy. Compare this to the fact that in 1967 a researcher went to Ghana to study asthma and found not one case, despite looking for three months.

Why the increase and why was Ghana free of asthma?

Pollen, dust mite faeces, and animal dander account for approximately 90% of allergies, the vast majority of allergic reactions.

Sensitivity to food and its additives, to insects, latex, chemicals, industrial ingredients, and particulates in the air is responsible for the majority of the remaining 10%. But, because a person can develop an allergic response to anything doctors can find identifying the cause of allergy symptoms in patients tough.

Why the immune system mounts an attack against seemingly harmless agents is the cause of some debate amongst researchers, the issue is far from settled.

When the immune system treats a harmless substance or agent as a threat, B lymphocytes (a type of white blood cell or macrophage) manufacture an antibody to attack it. The antibody, or cytokine, they produce is immunoglobulin E (IgE). IgE attaches to mast cells such as those present in the nasal and bronchial passages in staggering numbers when this happens (billions of molecules in a square millimetre of tissue), resulting in massive histamine release when the antibodies bind to the agent they are targeting and cause the mast cell to burst, releasing massive quantities of histamines. Inflammation and allergy symptoms follow in the familiar and miserable way.

Once sensitised, an individual's mast cells automatically release histamine when the invading agent reappears. Because IgE can sensitise cells anywhere in the body, individuals' symptoms vary (eg, a cat may produce hives in one person and generate sneezing or asthma in another person).

The Increase in Allergies

That genetics plays a role in an individuals susceptibility to allergies and ot autoimmune disorders is well established. But a shift in the human gene pool is not the explanation for the increased prevalence of allergies because the change in the incidence of these diseases in the population has occurred in such a brief, genetically speaking, period of time.

Shifts in population-based traits require several generations to emerge; yet as is obvious from the statistics quoted at the beginning of this page we have seen massive increases in the incidence of these diseases within a decade or two. Crohn's disease was unknown until just before World War II for instance, but today affects 1.4% of the US population, or half a million Americans. In one study that illustrates how rapid this change in our reactions has been, for example, frozen blood samples were randomly selected from syphilis screening initiatives conducted in 1987 and compared with those randomly selected from a 1998 HIV screening. The presence of IgE to common allergens in the 1998 sample was double that of the earlier sample.

Two of the numerous theories proposed have empirical support, that is they are backed up by observation and testing, by research. They are the allergen exposure hypothesis and the hygiene hypothesis.

Allergen Exposure Hypothesis

Simply stated, increased exposure to allergens leads to increased sensitivity. Today, people eat diverse and exotic foods, many containing artificial preservatives and additives; spend more time indoors, increasing contact with dust, synthetic materials, and household chemicals; are more likely to maintain pets indoors; and are exposed to more pollution. The exposure theory is supported by the increased prevalence of allergies primarily in developed countries, and mostly in urban environments, as opposed to rural communities. This seems on the face of it to be true, however many of these factors have been true in at least pockets of the population for longer than many of our immunological disorders and allergies manifested. So this cannot be the whole explanation.

Hygiene Hypothesis - Why Ghana was free of asthma

This theory proposes that inadequate exposure to infectious organisms leads to immune dysfunction. Under normal circumstances, the immune system is exposed to various viral, bacterial, fungal, or parasitic challenges and becomes properly calibrated to these real threats following successful defences. Today's public health successes, coupled with the increased use of antibiotics and vaccines, minimise opportunities to mount successful attacks against genuine attackers. IgE is still present, however, and, in the absence of harmful agents, it begins attacking harmless environmental substances.

There is an enormous amount of scientific evidence for this. Children who have had early infections manifest far less atopy, allergy and autoimmune disease. Populations in which parasitic infestation is common manifest lower levels of hay fever and asthma, or as in the case of Ghana in 1967 none that could be found. In the 60's a study in Israel correlated levels of home hygiene with MS in the adult children growing up in those homes. The cleaner the home the more likely the adult child was to develop MS. People who have had measles have fewer allergies, as do the younger children with multiple siblings, presumably because they have experienced far more childhood diseases because of exposure through their siblings.

Animal studies also support the hygiene theory; mice treated with antibiotics showed greater sensitivity to mould allergens, lab animals kept in sterile conditions develop Type I Diabetes and allergies. The hygiene hypothesis, however, cannot satisfactorily explain the high prevalence of allergies in poor inner-city housing, where potential exposure to harmful agents is increased. Similarly, total antibiotic sales do not correlate with shifts in the prevalence of allergies or autoimmune diseases.

So, for the Ghanaians of 1967 they were free of asthma because they lived and grew up in an environment much closer to the one we evolved in than that found in the sterile west. One filled with childhood disease, free of antibiotics and vaccinations and one in which parasites are common.