Last May, as Ebola crept across West Africa, America's top infectious disease expert told a group of Harvard students in a commencement speech to always second-guess their assumptions because "overconfidence can kill." Five months later in a hearing room on Capitol Hill, Tom Frieden was accused of not following his own advice - repeatedly assuring the nation it was safe from an Ebola outbreak even as two U.S. nurses became infected and one was allowed to board a commercial airline, each following safety protocols Frieden helped put in place.