The hygiene hypothesis: identifying microbial friends and protecting against microbial enemies

We frequently read about how we have 'become too clean for our own good' - but is this really true? What is meant by this anyway? And, more importantly, what are the implications for hygiene and public health? This issue has been reviewed in a recent report by the International Scientific Forum on Home Hygiene (IFH). ^sup 1,2^ Sally Bloomfield, Chairman of the International Scientific Forum on Home Hygiene and Visiting Professor, London School of Hygiene and Tropical Medicine, takes a closer look at the discussion.The hygiene hypothesis dates back to 1989, when Strachan proposed that a lower incidence of infection in early childhood could be an explanation for the rapid 20th century rise in allergic diseases. Over the last decade, researchers have recognised that the concept of a link between microbial exposure and allergy is probably correct, but the idea that children who have more infections are less likely to develop allergies is wide of the mark. Their other finding, which highlights the growing importance of this issue, is that the concept applies to a much broader range of chronic inflammatory diseases (CIDs) such as type 1 diabetes and multiple sclerosis, and also some types of depression and cancer.In 2003, Graham Rook proposed the 'Old Friends' hypothesis, which seems to offer a more rational explanation. He argues that the vital exposures are not colds, influenza, measles and other common childhood infections which have evolved relatively recently over the last 10,000 years, but rather the microbes already present over 2 million years ago in hunter-gatherer times when the human immune system was developing. His argument is that we have become so dependent on our 'Old Friends' that our immune systems cannot function properly without them.What sort of microbes do we need and when? Rook suggests that our 'Old Friends' are most likely to include the following:* Environmental species which inhabit our indoor and outdoor environments;* The normal microbiota of the human skin, gut and respiratory tract, and that of the animals we live with;* And organisms such as hepatitis A virus and helminths (worms) which establish chronic infections or carrier states, and that need to be tolerated.Studies now show how exposure to these microbes is vital because they interact with regulatory systems which keep our immune systems in balance. Without this our immune systems may react inappropriately, sometimes overreacting and causing asthma and hay fever, and sometimes attacking our own tissues and causing autoimmune diseases such as multiple sclerosis. Increasingly, it seems that diversity of exposure to these microbes is key to building a well-regulated immune system.For allergic disease, it seems that the most important times for exposure are early in development, during pregnancy and the first few days or months of infancy, and this exposure needs to be maintained over a significant period. This fits with evidence that Caesarean section may be associated with an increased tendency to develop allergies, while breastfeeding for 6 months or more can be protective. What we do not know is the extent to which exposures need to be maintained during childhood and adult life and whether these diseases could be contained or reversed by ongoing exposure. There is some evidence for this, but much more research is required to better understand.Are other factors involved? It now seems clear that the risk of allergic and other CIDs also depends on factors such as changes in diet, pollution, physical activity, obesity, socio-economic factors and stress. Genetic predisposition is also an influencing factor, which could explain why we do not all suffer from these diseases. We all still get some microbial exposure, which for some is sufficient, for others not. For these people, allergies and other CIDs may be triggered, when these other factors cause further immune dysregulation. …