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Is our zeal for cleanliness making us ill?
The Hygiene Hypothesis, formulated by the epidemiologist David Strachan about 20 years ago, argued that children's immune systems were not being sufficiently challenged - because of falling family size and increasingly sterile homes - to learn how to fend off diseases. The result was that once harmless invaders, such as cat hair, triggered immune overreactions (this is what constitutes an allergy). In the late Nineties, the evidence for Strachan's hunch was snowballing: kids in daycare showed lower rates of asthma than infants kept at home, suggesting that immunity might be conferred by early contact with other children.
But in recent years there has been a backlash against the Hygiene Hypothesis, especially from experts on infectious diseases. They worry be-cause the hygiene hypothesis lulls people into thinking that poor hygiene is OK, or beneficial, when the opposite is true. Poor hygiene allows bad germs to flourish, and the prevalence of gastrointestinal infections and MRSA, along with norovirus, show we should not drop our guard.
Professor Sally Bloomfield, an expert on infectious diseases at the London School of Hygiene and Tropical Medicine, is one who finds the persistence of Strachan's unproven thesis counterproductive. “When we have unacceptable levels of gastrointestinal disease, norovirus - and Sars and a possibile flu pandemic - the idea that hygiene is unnatural is frightening. We need to support cleanliness and hygiene. I still find people who think it's proven that we are too clean. We must dispel this.”
How has this error come about? Bloomfield says that while exposure to microbes seemed pivotal in the prevention of allergies, Strachan went farther, suggesting that it was disease-causing microbes (pathogens) that offered protection: “He made the link between exposure to infection and protection from allergy, but it could be benign microbes, rather than disease-causing ones, that are providing protection. It could be that as we've improved water and food, knocked out the benign bacteria along with the pathogens. Or it could be nothing to do with microbes.”
Bloomfield is a member of the International Scientific Forum on Home Hygiene, which suggested several years ago that the Hygiene Hypothesis be renamed the Microbial Exposure Hypothesis. It would convey the growing conviction that it is our modern relationship with microbes, rather than extra cleanliness, that is making us ill. Scientists are warming to the idea that the benign microbes colonising our guts and skin, rather than full-blown diseases, defend against allergy. A recent study published in the Journal of Allergy and Clinical Immunology showed that British, Swedish and Italian newborn babies with a narrow range of bacteria in their stools are more likely to have developed eczema at 18 months than newborns with a wider range of bacteria (researchers, from Lund University, Sweden, speculate that antibiotics given during delivery might be killing off beneficial bacteria).
Another way of smuggling “good” microbes back into the body is to consume probiotic yoghurt drinks or fruit juices. These products contain so-called friendly bacteria. But the science on whether they improve health remains contradictory.
And that's the rub: we don't yet know if extreme hygiene has propelled the rise of allergies. What we do know, to our cost, is that a lack of cleanliness is leading to an explosion in preventable, transmissible infections at home and in hospitals.