Ebola virus infects a third American health worker who tested positive for the virus while working with patients in West Africa, the Christian missionary group SIM said Tuesday.
The group did not release the name of the doctor, but said he had been working in obstetrics at the SIM-funded ELWA hospital in the Liberian capital Monrovia.
Two Americans who also worked at ELWA, Doctor Kent Brantly and nurse Nancy Writebol, were previously flown home from Liberia and successfully treated for the virus.
Unlike Brantly and Writebol, the new US victim had not been working directly with Ebola patients, and it is not yet clear how he caught the disease, which is usually fatal.
The doctor has been isolated in the hospital's dedicated Ebola unit.
"My heart was deeply saddened, but my faith was not shaken, when I learned another of our missionary doctors contracted Ebola," said SIM president Bruce Johnson.
"We have gifted Liberian doctors, medical staff and support staff who are carrying on the fight."
Over the past six months the world's worst ever Ebola outbreak has killed 1,552 people in several West African countries and infected 3,062, according to the latest figures released by the WHO.