Another American doctor has been diagnosed with Ebola, the third to contract the virus in West Africa since the outbreak began in March.

The doctor, who has not yet been named, was working for the missionary group SIM USA and was treating pregnant women in the Liberian capital of Monrovia, according to a report from Fox News.

SIM USA said the doctor was not treating Ebola patients in the hospital's isolation unit, and that it's not known how he was infected. The doctor isolated himself as soon as he developed Ebola symptoms and is reported to be doing well.

"My heart was deeply saddened, but my faith was not shaken, when I learned another of our missionary doctors contracted Ebola," said Bruce Johnson, president of SIM USA. "We have gifted Liberian doctors, medical staff and support staff who are carrying on the fight."

This doctor is the second from SIM USA to contract Ebola working in Liberia. Nancy Writebol caught the virus in July and was flown to the U.S. to receive treatment, USA Today reported. She was given experimental drug ZMapp, and has recovered and has been released from the hospital.

Another doctor, Kent Brantly, contracted Ebola in Liberia working for Samaritan's Purse. He also was flown to the U.S. and received ZMapp. He has recovered and recently gave an interview to Matt Lauer about his experience.

"I felt like I was about to die and I said to the nurse who was taking care of me, 'I'm sick, I have no reserve and I don't know how long I can keep this up," Brantly said.

Including the three American doctors, more than 240 aid workers have been diagnosed with Ebola in the West African outbreak, and more than half have died, according to World Health Organization numbers.

Since the March outbreak that started in Guinea, Ebola has infected more than 3,000 people and killed more than 1,500. It is the largest Ebola outbreak on record.