The Ebola outbreak in the Democratic Republic of Congo may have begun as early as February and may exceed 20,000 cases over the next three months, according to modeling published on Friday by the Centers for Disease Control and Prevention. Still, the overall risk to Americans over that period is low, the agency said in a second assessment , noting that previous large outbreaks have resulted in very few cases in the United States. The C.D.C. published both reports in its flagship journal, the