Changing exposure to intestinal helminths, or alterations in our intestinal microbiome, have been independently proposed to underlie the increasing incidence of chronic inflammatory diseases including allergy, autoimmunity and inflammatory bowel disease (IBD) observed in developed nations. A recent study in Science links these findings by showing that intestinal helminth infection can prevent the outgrowth of a common intestinal bacterium that causes IBD in genetically susceptible mice.