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About seven years ago, Aaron Silverbook and his then-girlfriend, a biologist, were perusing old scientific literature online. “A romantic evening,” joked Silverbook. That night, he came across a study from 2000 that surprised him. Scientists had genetically engineered an oral bacterium that they said could possibly prevent tooth decay: “I read it and sort of boggled at it and said, ‘Wow, this is a cavity vaccine. Why don't we have this?’”
So...