Old Friends Hypothesis


Last modified

18 April 2020

The Old Friends Hypothesis attempts to explain how modern hygiene practices have led to a rise in inflammatory diseases such as asthma, hayfever, and food allergies to things like peanuts, lactose and gluten.

My interest in hygiene started when I was 8. Walking home from school with Joey I blurted out, “I have a bath once a week and use the same water that my younger sisters bathed in.”. He made me feel deep shame that I didn’t use fresh water nor shower daily.

Two years later, in 1989, David Strachan put forward the Hygiene Hypothesis. He noted that a hundred years ago it was known that hayfever was less common in farmers than those in cities. That the number of people with hayfever has been increasing ever since. He suggested that due to decreasing family size our immune systems weren’t being stimulated to become strong. The theory is weak because it suggests that exposure to any disease is good. It is true that some humans can tolerate some nasty bugs indeed. For example consider the river Ganges. It is one of the dirtiest rivers on Earth. It starts in the Himalayas and passes almost 400 million people by the time it gets to the holy city of Varanasi. It receives not only their raw sewerage but the bodies of the dead who are cremated on it’s edge, along with whole bodies of those unfit to be burnt. It has a faecal coliform content of 1.5 million per 100ml… anything over 500 is considered bad. For the Hindu to bathe in this river is an act of religious devotion. There are those with white hair, or none, who have bathed in it every day. It would be easy to take the theory and combine it with knowledge of Hindu custom and say that we should actively expose ourselves to disease. This is false… but that hasn’t stopped from people getting very sick because they dropped hygiene standards.

To be clear. It is good practice is to wash our hands after going to the toilet, after handling animals, after contact with the sick, and before eating or preparing food.

Old Friends Hypothesis addresses the flaws in Strachan’s theory. It divides diseases into those that kill us quickly and those that kill slowly, or that we can have without knowing. E.g. some strains of flu can kill, but Chlamydia can be carried for months or years without symptom. Chlamydia is not just a pretty flower. The divide started 10,00 years ago with the advent of agriculture. Prior to that humans lived in small family groups with a territory for hunting and gathering. They might not see another group for weeks or month. So if Craig the Caveman catches a sniffle and dies of the flu then the virus dies with him. But if he doesn’t know he has galloping knob rot then he may pass it on when he meets a girl at the end of year tribal gathering.

Graham Rook proposed the “Old Friends Hypothesis” in 2003. It suggests that as mammals evolved over the last 160 million years that their bodies learnt to deal with the microbes around them. If we were to count the number of microbes on us and in us we’d find that they outnumber our own cells. He suggests that we require exposure, or colonisation, by these old microbes to stimulate and maintain a healthy immune system. We have beaten infectious diseases such as the ones that involve diarrhoea and vomiting due to modern plumbing and weekly rubbish pick up but in becoming cleaner we are now seeing higher rates of asthma, hayfever, and food allergies. Further research is need to apply this knowledge to make society healthier.

What we do know is that those who spend time outdoors and around animals tend to enjoy lower rates of these modern diseases. It is easy today for people who live in cities to spend most of their lives indoors, or outdoors on the deserts that we call footpaths, roads and chlorinated pools. We may well benefit from visiting parks, getting into the dirt whilst gardening, and swimming in rivers and the sea.

Some companies are applying this theory by creating a new type of deodourant. They don’t have strong scientific evidence for it, but it does sound plausible. Their idea is this… Armpit odour is the waste product of a bacteria that eat our sweat. If we spray ourselves with a bacteria that eats the waste or replaces the bad bacteria then we won’t smell. For now I’m sticking with Old Spice. But it is interesting microbes may be able to help us stay healthy.

I’ll leave you with a personal anecdote which supports the benefits of being outdoors.  My wife Jenima has commented that when she picked me up after a week long Army exercise outdoors without showering that I did not have any noticeable body odour.


Old Friends Hypothesis (Plain English synopsis of theory)

Microbial ‘Old Friends’, immunoregulation and stress resilience

99th Dahlem Conference on Infection, Inflammation and Chronic Inflammatory Disorders: Controversial aspects of the ‘hygiene hypothesis’

The Ganges: holy river from hell

New generation of deodorants 'on the way'

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