One of the slightly odd things that emerges when one looks at the history of health problems in the developed world is that, although the incidence and severity of infectious diseases, such as tuberculosis, has steadily declined, the incidence of allergic and auto-immune conditions, such as eczema and asthma, has increased. In some cases the increase is quite significant: in developed countries, where many childhood diseases have been largely eliminated , the rate of asthma is ~10%., whereas in the 19th century, in the same countries, asthma was a very rare disease.
One of the possible explanations for this, first suggested in 1989 by David Strachan in the British Medical Journal, is the hygiene hypothesis. Briefly, the hypothesis is that the human immune system evolved in an environment in which infants and small children were routinely exposed to a variety of micro-organisms: pathogens, symbionts, probiotics, and so on; and that the immune system needs that sort of exposure to “calibrate” itself, lest it over-react to basically benign substances. In the developed world, we now live in very clean environment, by evolutionary standards, and children are exposed to many fewer organisms. There is some evidence to support this hypothesis: children that are from large families or families that have domestic animals exhibit lower rates of asthma than only children. Similarly, the rates of allergic and auto-immune disorders are much lower in poor, developing countries, where hygiene is generally poorer. A relatively recent study, reported by the BBC, found that exposure to some bacteria actually enhances the skin’s ability to heal itself:
Children should be allowed to get dirty, according to scientists who have found being too clean can impair the skin’s ability to heal.
Normal bacteria living on the skin trigger a pathway that helps prevent inflammation when we get hurt, the US team discovered.
Another new paper, published online this week in the Proceedings of the Royal Society B (abstract), and reported in articles at Live Science and Science Daily, suggests an additional reason why “germs may be good for you”. The research team, headed by Thomas McDade of Northwestern University, found that lack of exposure to micro-organisms in early childhood was associated with higher levels of C-reactive Protein [CRP] in adulthood. High CRP levels are associated with increased levels of inflammation, and appear to be a risk factor for cardio-vascular disease. Since inflammation is an innate response of the immune system, the researchers hypothesize that early exposure to relatively benign organisms helps train the immune system to avoid over-reacting.
“Contrary to assumptions related to earlier studies, our research suggests that ultra-clean, ultra-hygienic environments early in life may contribute to higher levels of inflammation as an adult, which in turn increases risks for a wide range of diseases,” said Thomas McDade, lead author of the study, associate professor of anthropology in Northwestern’s Weinberg College of Arts and Sciences and a faculty fellow at the Institute for Policy Research.
Now, of course, this all has to be balanced against the fact that better hygiene and public sanitation have been an enormous boon to human health. Mortality among infants and small children from diarrhea-causing infections is still a severe problem in the developing world, and only a lunatic would suggest that infecting oneself with Y. pestis or B. anthracis would be a good idea. But perhaps some first-world parents can afford to relax just a bit, and let the kids play with Fido or Fluffy without the biohazard suits.