Are our efforts to improve hygiene reducing people’s immunity or causing the rise in allergies?
No, on both counts.
As regards immunity to infection, it’s true of course that infection can convey some immunity: becoming ill with specific diseases such as chicken pox or measles conveys lasting immunity to that specific disease for example. But it doesn’t make the person more resistant to salmonella, flu or athlete’s foot. Even with flu, catching flu one year only gives limited immunity to next year’s strain.
There’s no known mechanism through which repeated infections would strengthen the immune system in some general way. If that were the case, then instead of enjoying constantly increasing levels of health and life expectancy as we've improved defences against infectious disease over the last century, we’d have started to slip backwards as our immune systems broke down and common infections became rampant. There’s no evidence we’re any more susceptible to common infections now than we ever were.
This idea is fundamentally a confusion arising from the ‘hygiene hypothesis’ — the idea that allergies have increased because children suffer fewer infections.
The Hygiene Hypothesis
The idea that people today are living in a ‘super-clean’ world where few microbes are to be found is simply a myth. Homes, schools, workplaces, almost everywhere we go (and indeed our bodies) abound with all kinds of microbes.
It’s also a myth that there is some clear scientific evidence that shows the more hygienic someone is, or the cleaner their home, the more likely they are to be allergic or have allergic children. But it is very clear that the less hygienic they are the greater risk they run of infectious disease, which can have serious consequences. It’s essential not to get complacent about hygiene.
So having set that straight, why are so many more people allergic, and could the extent of our contact with microbes have anything to do with it?
The rise in allergies has happened over several decades and is mainly in ‘atopic’ allergies, which are not caused or triggered by specific substances, and in some food allergies. Atopic allergies include hay fever, much childhood eczema and asthma.
Scientists have been looking for reasons for the rise in atopic allergies for 20 years and more. It seems to be broadly connected with ‘western lifestyle’ but all that’s really clear is that many different factors are likely to be involved. The things still being investigated as possibly part of the explanation range from diet to diesel fumes, paracetamol to pollutants and genetics to lack of exercise.
As regards microbes, there’s a growing consensus that losing touch with certain microbial ‘old friends’ that our immune systems evolved with back in the stone age is likely to be one factor in the rise of allergies, but potentially also of other immune-related diseases such as Type 1 diabetes.
Since the 1800s when allergies began to be more noticed, the mix of microbes we’ve lived with, and eaten, drunk and breathed in has been steadily changing. Partly this has happened through deliberate measures to protect us from infectious disease – through antibiotics, vaccines, clean water and clean food as well as personal and domestic hygiene. Partly it’s happened through incidental lifestyle changes such as the move from rural to urban living, and more indoor vs outdoor play.
Modern homes, schools and workplaces have a different and less diverse mix of microbes than their rural equivalents of the past. This is nothing to do with cleaning practices: apart from the highly-controlled environments of operating theatres and clean-rooms, even the cleanest-looking places still abound with bacteria, viruses, fungi, moulds and dust mites. The change in the mix of microbes has come about mainly because microbes come in from outside and the microbes in towns and cities are very different from those on farms and in the countryside.
Assessing all the evidence, it seems very unlikely that everyone needs to get infected, with possibly fatal consequences, to get protected against allergies. But it’s distinctly possible that changed exposure to microbes of some kind is a factor. Growing up on a farm, for example, does seem to reduce the risks of having allergies as a child. But no-one’s clear yet exactly what it is about the farm that’s protective.
The ‘old friends’ we seem to have lost touch with are mainly harmless to healthy people. So there’s no stark choice between rising allergies and immune disease or rising infectious disease. How we begin to reverse the trend in allergies isn’t yet clear but it won’t be achieved by relaxing hygiene. That will only expose people to old and new enemies like E. Coli O104.
It’s unfortunate this idea was called the Hygiene Hypothesis, because it’s not really about ‘hygiene’ as we practise it today: the ‘microbial exposure’ or ‘Old Friends’ hypothesis expresses it much better. Hygiene professionals, including the originator of the hypothesis, became very concerned that stories about the supposed dangers of ‘being too clean’ were confusing people and undermining our protection against infectious disease. You can read their assessment and advice here.
A recent review of the current science has been compiled by the International Scientific Forum on Home Hygiene (IFH). A summary is available here.