Clostridium difficile bacteria produce a toxin that damages the intestine and causes severe diarrhea. Courtesy of David Goudling/Nature Genetics. hide caption

itoggle caption Courtesy of David Goudling/Nature Genetics.

Childhood obesity is on the rise in many countries and overuse of antibiotics is now on the radar as a possible factor in the epidemic. Here 18-month-old twins are weighed in a nutritionist's office in Colombia. Raul Arboleda/AFP/Getty Images hide caption

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The FDA's latest effort to end the use of antibiotics as growth promoters in animals is getting mixed reviews from activists. Rob Carr/AP hide caption

itoggle caption Rob Carr/AP

Pigs take a mud bath at the De Jofrahoeve pig farm in Esch, Netherlands. Dutch farmers treat their animals with almost three times the antibiotics that their Danish neighbors use. Robin Utrecht/AFP/Getty Images hide caption

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Many livestock groups say there's no evidence that antibiotics in livestock feed have caused a human health problem, but researchers beg to differ. Scott Olson/Getty Images hide caption

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Adrian Mesa protests the overuse of antibiotics in meat production outside a Burger King in Coral Gables, Fla. in 2003. Joe Raedle/Getty Images hide caption

itoggle caption Joe Raedle/Getty Images