An old joke holds that the only people allowed to refer to themselves as “we” are royalty, editors and people with tapeworms. Yet as a 2007 NEWSWEEK cover story notes, we are all collections of thousands of species of bacteria, worms and other parasites—and losing some of them, it turns out, has dire consequences.

Helicobacter pylori, which lives in the acid environment of the stomach, can cause both gastric ulcers and stomach cancer. But it also seems to protect against esophageal reflux and cancer of the esophagus, and now that it is on the verge of extinction in the West, report Martin Blaser and Yu Chen of NYU in the Journal of Infectious Diseases, watch out for an explosion of asthma.  

H. pylori infected around 90 percent of children born at the turn of the 20th century but fewer than 10 percent now, mostly thanks to better hygiene and widespread use of antibiotics. But the bacteria had evolved the ability to calm the human immune system. Remove the bacteria and immune reactivity can overcompensate. One result may be that asthma, a hyper-reactive immune response in tissues lining the airways, has spread like a modern plague.

In their paper, Blaser and Chen review 12 studies on the relationship between infection with H. pylori and several immune-mediated diseases, including asthma, hay fever, eczema and other skin rashes. Message: the lower the infection rate with H. pylori, the higher the incidence of immune diseases. Blaser also found the same inverse relationship between H. pylori and asthma and skin rashes.

Worms, too, can damp down humans’ immune reactions—far enough that the parasites can live in the gut, but not so far that the host is defenseless against other threats. Without the calming effect induced by gut worms, the immune system becomes over-active, as a story in The New York Times magazine shows. To summarize, one result might be an increase in inflammatory bowel disease (IBD) and other disorders (like arthritis) caused by an over-active immune system that attacks the body’s own tissues.

When Joel Weinstock, an Iowa gastroenterologist, infected volunteers—patients with Crohn’s disease—with parasitic intestinal worms, 23 of 29 improved after 24 weeks; 21 were in complete remission. In a second study, 13 of 30 patients with ulcerative colitis who were infected with worms got better, while only 4 of 24 controls (given a placebo) improved.

All of which suggests that our never-ending quest to rid the world of microbes (that means you, anti-microbial soap user) will have unintended consequences--and not necessarily happy ones.