Another U.S. missionary doctor contracts Ebola in Liberia

Another American doctor has been infected with the Ebola virus in Liberia, the missionary group he worked for, SIM, said Tuesday.

The doctor, whose name was not released, was working in the obstetrics unit at the SIM ELWA hospital in Monrovia, Liberia. The group says he was not involved in treating Ebola patients and it is not clear how he became exposed to the virus.

He is now receiving treatment at the hospital's Ebola isolation unit. In a statement, SIM said he "is doing well and is in good spirits."

Last month, Dr. Kent Brantly and Nancy Writebol, American aid workers who were infected with Ebola, were discharged from an Atlanta hospital after recovering from the virus. Writebol also worked for SIM.

The Ebola outbreak, centered in the West African nations of Liberia, Guinea, Sierra Leone and Nigeria, has killed more than 1,500 people since March. The World Health Organization says the spread of the disease continues to accelerate. Health care workers and family members of Ebola patients are especially at risk since the virus spreads through direct contact with bodily fluids from an infected person.

There is currently no vaccine or proven treatment for Ebola, although an experimental serum called ZMapp appeared to help Brantly and Writebol recover and has tested effective in primates. A very limited supply of the drug has since run out.

This week, researchers from the U.S. National Institutes of Health are beginning the first human clinical trial to test the safety of a possible vaccine, a process that will take months.