BUG ON BUG: “Studies show colonization with a non-disease-causing strain of C. difficile can prevent infection by the more dangerous strains. Patients who are not colonized by benign strains may risk become infected by the harmful strains. The harmless strains occupy the same niches in the intestine that the infective strains would take if the harmless strains weren’t there.””
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http://www.eurekalert.org/pub_releases/2013-10/asfm-bvb100213.php