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Modern disease is often caused by a lack of parasites

My latest Mind and Matter column in the Wall
Street Journal is a review of a remarkable new science book:


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

By Matt Ridley | Tagged:  rational-optimist  wall-street-journal