Another American missionary doctor has tested positive for Ebola in Monrovia, Liberia. He is the third American health care worker to contract the virus.

The name of the doctor, who worked for the Christian aid group SIM, has not been released. But the group said in a statement that he did not treat Ebola patients. Instead, he saw obstetrics patients at the organization's main ELWA hospital, which is separate from its Ebola isolation unit. How he became infected is unknown.

The doctor isolated himself at the onset of symptoms and is currently being treated at the isolation facility, according to the group. They added that he is "doing well and is in good spirits."

"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, in the statement. SIM was founded in 1893 as the Sudan Interior Mission; the group has since expanded its scope and adopted the slogan "Serving In Mission."

SIM will hold a press conference Wednesday at its Charlotte, N.C., headquarters to provide more details.

The new case comes less than two weeks after the first American patients were discharged Ebola-free from a hospital in Atlanta. Dr. Kent Brantly and Nancy Writebol, who were working with Samaritan's Purse and SIM in Monrovia, had each been given an experimental Ebola drug during their hospitalization.

The current outbreak is the largest since the virus's discovery in 1976, with cases in Guinea, Liberia, Sierra Leone, Nigeria and, as of last week, Senegal. So far, there are more than 3,000 cases and 1,500 deaths, according to the World Health Organization, which also said the numbers are vastly underestimated.

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