Discover how quickly and effectively some strains of bacteria can evolve resistance to drugs prescribed by doctors to kill the bacteria and the implications of antibiotic resistance to human health, in this video from NOVA Wonders: What’s Living in You? While evolution by natural selection may take place over millions of years in the animal kingdom, natural selection in the world of microbes happens quickly—producing strains of antibiotic-resistant bacteria in several hours or even minutes. Using a growth plate to demonstrate this idea, one scientist explains that even progressively stronger drug doses do little more than delay the appearance and growth of new antibiotic-resistant strains. As antibiotics are being used to target Clostridium difficile (C. diff) and other harmful bacteria in humans, they may be doing more harm than good by killing off good bacteria in our bodies. This resource is part of the NOVA Wonders Collection.