In studies of what is called the hygiene hypothesis, researchers are concluding that organisms like the millions of bacteria, viruses and especially worms that enter the body along with “dirt” spur the development of a healthy immune system. Several continuing studies suggest that worms may help to redirect an immune system that has gone awry and resulted in autoimmune disorders, allergies and asthma.

From the archive:

Comparison of the prevalence of I.B.D. and surveys of worm-infestation rates revealed a telling pattern. About 10 years after improved hygiene and deworming efforts reduced worms in a given population, I.B.D. rates jumped. Weinstock had his hypothesis: after a long coevolution, the human immune system came to depend on the worms for proper functioning. When cleaner conditions and new medicines evicted the worms from our bodies, the immune system went out of kilter. “Hygiene has made our lives better,” says Weinstock, now at Tufts University. “But in the process of eliminating exposure to the 10 or 20 things that can make us sick, we’re also eliminating exposure to things that make us well.”

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