LONDON—A black market for an Ebola treatment derived from the blood of survivors is emerging in the West African countries experiencing the worst outbreak of the virus on record, the World Health Organization said.

The United Nations health agency will work with governments to stamp out the illicit trade in convalescent serum, WHO Director-General Margaret Chan told reporters Friday in Geneva, where the organization is based. There is a danger that such serums could contain other infections and wouldn’t be administered properly, Chan said.

The WHO is encouraging the use of properly obtained serum to treat current patients and said last week it should be a priority. A third U.S. missionary worker who was infected with Ebola in Liberia and flown to the U.S. for medical care was treated with blood transfusions from another American who recovered from the virus last month. Doctors hope the virus- fighting antibodies in the blood help the 51-year-old physician, Rick Sacra.

More than 300 health-care workers have been infected with the Ebola virus, and almost half of them have died, the WHO said in a situation report Friday.

The blood of survivors has natural antibodies against Ebola. Antibodies are produced by white blood cells and bind to foreign invaders like viruses or bacteria, either neutralizing them or flagging them for destruction by other parts of the immune system. About half of the people infected during the current outbreak have survived, providing a potential pool of donors.

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