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An American aid worker in West Africa has tested positive for Ebola, his organization announced Tuesday.

SIM USA said the doctor immediately quarantined himself once he began to show symptoms of Ebola and is "in good spirits" in the organization's Ebola isolation unit.

The doctor was working in SIM's hospital facility in Liberia—the epicenter of the current Ebola outbreak—but was not treating Ebola patients, SIM said in a statement. He was at a separate hospital on the same 130-plus-acre campus, and the organization does not know how he contracted Ebola.

SIM did not release the doctor's name.

The doctor is the third American to be infected with Ebola during this outbreak. Two other U.S. aid workers—including Nancy Writebol, who also worked with SIM—were successfully treated in the U.S. last month, the first time Ebola patients had entered this country.

The Ebola outbreak in West Africa has killed more than 1,400 people, including more than 120 health care workers.

Also on Tuesday, the Health and Human Services Department announced a partnership to speed the production of ZMapp, an experimental Ebola treatment. The government will provide roughly $25 million to help the drug's manufacturer, Mapp Biopharmaceutical, scale up production and begin clinical trials to test ZMapp's safety and effectiveness.

The two U.S. aid workers who overcame Ebola were both treated with ZMapp, but because the drug has never been tested in humans, scientists don't know what role -- if any -- it played in their recovery. Mapp sent its entire ZMapp supply to Africa, which is why the government is stepping in to help the company quickly make more of the drug so clinical tests can begin.

-- This story has been updated.

This article is from the archive of our partner National Journal.

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