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.

“We’re hoping it jump starts his immunity,” Phil Smith, medical director of the biocontainment unit at the hospital in Omaha, Nebraska, where Sacra is being treated, said Thursday during a conference call with reporters. “To survive you have to build up enough antibodies to neutralize the virus. We’re hoping to buy him some time, in other words, to give him antibodies to help his immune system battle the Ebola virus and let him get ahead of the curve.”

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.

Sacra was admitted to the Nebraska Medical Center in Omaha on Sept. 5. A day later, he got the first of two blood plasma transfusions from Kent Brantly, the 33-year-old missionary doctor successfully treated for the deadly disease after being evacuated from Africa.

Sacra was infected while delivering babies in Monrovia, Liberia, on behalf of the Christian missionary group SIM USA. He also has been receiving an experimental drug every night for the last several days, said Angela Hewlett, associate medical director of the Nebraska hospital’s biocontainment unit.

The Nebraska doctors have been asked not to disclose which experimental drug is being given each day to Sacra, Smith said. There isn’t enough information available on its benefits, there is a very small supply and the doctors don’t want to encourage the belief that it may be a cure-all, he said.

“We don’t know if this is having an effect at all,” Smith said. “We just administered everything we had access to, honestly.”

Sacra is stronger and getting back to his normal self, Hewlett said. A relapse is less likely, though it’s impossible to know for sure, Hewlett said.

A fourth patient was taken to Emory University Hospital, where Brantly was treated, this week and is being treated in the isolation unit there. Authorities haven’t released any details on the patient. The U.S. State Department plans to evacuate any U.S. workers in Africa who become infected, according to an agency document.

The Ebola outbreak is the largest in history, sickening 4,784 people and killing more than 2,400 in four West African countries, Chan said today. In Liberia, almost 400 confirmed and probable cases have emerged in the past week, almost double the number of newly reported cases in the previous week, with the increase mainly in the capital, Monrovia, the WHO said.

“There has been no indication of any downturn in the epidemic in the three countries that have widespread and intense transmission,” namely, Liberia, Guinea and Sierra Leone, the WHO said in its report.

There are no approved drugs to treat Ebola. Patients are given intravenous fluids, blood transfusions and antibiotics to bolster their immune systems and help fight off other infections.

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.

The WHO is helping establish a system that can be used to safely draw blood from those who have recovered from the disease, prepare it and re-inject it into patients. Doctors at Emory and Nebraska are also working on lists of survivors by blood type who could donate.

The U.S. National Institute of Health is working on an Ebola vaccine and other treatments are in development by Tekmira Pharmaceuticals Corp., Fujifilm Holdings Corp., BioCryst Pharmaceuticals Inc. and Sarepta Therapeutics Inc.