When I was taking invertebrate zoology, my teacher remarked that if you got rid of all multicellular organisms, and replaced nematodes (tiny little, multicellular worms) with points of lights, you would see the outline of every multicellular organism on the planet. Since nematodes are everywhere, it’s surprising that the role of nematodes in the maintenance of ‘normal’ health hasn’t been well investigated. So this NY Times magazine article about nematodes and Crohn’s disease fascinating (all the more so since I just helped submit a proposal to study the effects of the microbiome on inflammatory bowel disease):

The prevalence of inflammatory bowel disease (I.B.D.) across North America increased markedly during the 20th century. Many thought that “bad” genes would eventually explain the spike, but Weinstock didn’t buy it. In areas where fewer than two generations ago the I.B.D. incidence might have been as low as 1 in 10,000, it was now 1 in 250. A defective gene couldn’t spread that quickly, he reasoned. It had to be something in the environment. But what? Stumped, Weinstock tried turning the question around. Instead of asking what triggered I.B.D., he asked what, before the 20th century, protected against it?

…These worms, or helminths, have a paradoxical effect on the host. Rather than induce inflammation, which is the body’s typical response to invasion, the intruders calm the host immune system. They force a peace, scientists think, to avoid eviction and keep the host–their home and food source–as healthy as possible. As Weinstock considered the I.B.D. puzzle, he wondered if immune manipulation by worms could incidentally protect against other diseases.

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.

The NY Times also has a related article on worms and allergies.