One of the most amazing (and controversial) things we have been doing for years is altering the genetics of organisms. This had led to breakthroughs in medicine (e.g. mass production of insulin, vaccines, etc.), agriculture (e.g. increase in food production, pest-resistant crops, etc.), energy (e.g. biofuels), and more.
Bacteria were the first to be modified in 1973. In the past decade, scientists at Johns Hopkins Kimmel Cancer Center have taken a toxic and possibly fatal bacteria (Clostridium novyi) and altered it to only devour tumors while leaving healthy tissue alone. To learn more check out Discover’s post titled Toxic Bacteria Devours Tumors With Precision.