Sep 11, 2012 1:35 PM GMT
Your great-grandparents faced infectious diseases that hardly threaten you today: tuberculosis, polio, cholera, malaria, yellow fever, measles, mumps, rubella, smallpox, typhoid, typhus, tapeworm, hookworm…. But there's also a long list of modern illnesses that your great-grandparents barely knew: asthma, eczema, hay fever, food allergies, Crohn's disease, diabetes, multiple sclerosis, rheumatoid arthritis. The coincidence of the rise in these "inflammation" diseases, characterized by an overactive immune system, with the decline of infection is almost certainly not a coincidence.
Natural experiments in recent decades support the idea that while modern hygiene defeats infection, it also promotes allergy and autoimmunity. Finns isolated in an impoverished Soviet province had more parasites and fewer allergies than Finns in Finland. Swedes in clean Stockholm had three times as much asthma as Estonians in smoky Estonia. Ethiopians and Gambians got allergies when they lost their intestinal worms. Growing up on a farm greatly cuts allergy risk.
In a remarkable new book, "An Epidemic of Absence," Moises Velasquez-Manoff draws together hundreds of such studies to craft a powerful narrative carrying a fascinating argument. Infection with parasites prevents or ameliorates many diseases of inflammation. The author briefly cured his own hay fever and eczema by infecting himself with hookworms—before concluding that the price in terms of diarrhea and headaches was too high.
I've touched on the "hygiene hypothesis" in these pages before. In its cartoon form the argument—that in a clean world our immune system gets bored and turns on itself or on harmless pollen—isn't very convincing. But Mr. Velasquez-Manoff makes a far subtler, more persuasive case. Parasites have evolved to damp our immune responses so that they can stay in our bodies. Our immune system evolved to expect parasites to damp it. So in a world with no parasites, it behaves like a person leaning into the wind when it drops: The system falls over.
Moreover, just as brains outsource much of their development to the outside world—the visual system is refined by visual input, the language system can only develop in a language-using society—so the immune system seems to have happily outsourced much of its regulation to friendly microbes. Without them, the immune system becomes unbalanced.
Timing seems to be key. If you pick up Epstein-Barr virus and Helicobacter early in life from your mother pre-chewing your food, they seem to help protect you against inflammation diseases. Catch them later and they may cause multiple sclerosis and stomach cancer, respectively.
One of Mr. Velasquez-Manoff's most surprising chapters is on autism, a disorder that almost exactly parallels asthma in its recent rise among affluent, urban, mainly male, disproportionately firstborn people. Better diagnosis explains perhaps half the rise, but the brains of people with autism are often inflamed, and there's anecdotal evidence that infection with worms or viruses can tame autistic symptoms, at least temporarily.
There's also a link between inflammation during pregnancy, caused by allergy or autoimmune disease (or chronic, low-grade infection), and autism in the child. Acute infections during pregnancy, on the other hand, correlate with schizophrenic symptoms, which may be why schizophrenia is growing rarer while autism grows more common.
Mr. Velasquez-Manoff even raises the possibility that heart disease, diabetes, obesity and even some kinds of cancer and depression may owe something to an unbalanced immune system caused by an impoverished microbial ecosystem. Few doctors are yet willing to recommend deliberate infection with parasites to regulate the immune system, especially not for pregnant women. But that we should all be rearing our kids to be a little bit dirtier—in a healthy, rural, probiotic sort of way—looks more and more like good advice.