Outnumbering our own cells more than 10 to one, the microbes thriving peacefully in the human body help keep us healthy. New research shows they also aid in the treatment of cancer. The two studies find in mice that gut bacteria bolster the effects of three antitumor regimens by priming the immune system. In each case, when a mouse's microbial residents are missing—as when antibiotics are given—the treatments are far less effective. Investigators warn against applying the mice findings to people, but already one of the researchers is being more cautious about prescribing antibiotics for her cancer patients.