GENEVA — The Ebola outbreak in West Africa is “unquestionably the most severe acute public health emergency in modern times,” Dr. Margaret Chan, the director general of the World Health Organization, said Monday.

Dr. Chan, who dealt with the 2009 avian flu pandemic and the SARS outbreaks of 2002-3, said the Ebola outbreak had progressed from a public health crisis to “a crisis for international peace and security.”