A U.S. citizen working for a humanitarian organization in Congo has tested positive for the Ebola virus, the U.S. Centers for Disease Control and Prevention said.
People have set fire to an Ebola treatment center in one of the towns at the heart of the outbreak in eastern Congo as fear and anger grows.
The outbreak has been declared a public health emergency requiring international response.
Dr. Stafford was exposed while treating patients in the democratic republic of Congo, where an Ebola outbreak has killed nearly 120 people.
The secretary's comments come after the CDC said a "small number of Americans" are directly affected by the current Ebola outbreak in the DRC.
The CDC said Monday it is preparing to restrict entry for travelers arriving from parts of central Africa where an Ebola outbreak has been declared.