Researchers are seeking to use the ability of the hookworm, Necator americanus, to suppress immune system responses that occur in conditions including Crohn’s disease, multiple sclerosis, asthma and other allergies. Recently-completed studies suggest giving worms to people with hay fever is safe, and potentially beneficial.
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Scientists have suggested a lack of such intestinal parasites has left those in the developed world more vulnerable to diseases caused an overactive immune system.

in 1999 the Dutch biologist Maria Yazdanbakhsh found that eliminating intestinal worms from infected children in African villages immediately predisposed them to allergies.”