President Obama vowed a more “aggressive” federal response to the spread of Ebola in the United States. And he offered assurance tonight that a “serious outbreak” remains extremely unlikely. “I want people to understand that the dangers of you contracting Ebola, the dangers of a serious outbreak, are extraordinarily low,” he said after meeting with Cabinet members and other top aides. “But we are taking this very seriously at the highest levels of government.” That assurance that a “serious” outbreak is unlikely is markedly less optimistic than the one Obama offered only a week earlier in a call with governors...