Tom Frieden (L), director of the U.S. Centers for Disease Control and Prevention (CDC), speaks during a hearing on combating the Ebola threat, on Capitol Hill in Washington D.C., capital of the United States, Aug. 7, 2014. CDC Director Tom Frieden said Thursday he has activated the level of the agency's response to the west African Ebola outbreak to its highest alert status.(Xinhua/Bao Dandan)