For years, public health experts warned of the possibility of an illness spreading across the globe and killing millions. After all, it had happened before. Measures were in place around the world to spot early signs of a never-before-seen bug with dangerous potential. That's why on Dec. 31, 2019, China notified the World Health Organization that a novel pathogen was circulating in Wuhan, the most populous city in central China. A day later, in that city of 12 million, a wet market selling live animals was shuttered because of fears it was the source of the virus that would later…