By Andrea Lorenz

CHARLOTTE N.C. (Reuters) - A third U.S. missionary infected with the Ebola virus in Liberia was identified on Wednesday, even as a fellow aid worker with the same Christian organization publicly shared her battle with the deadly virus for the first time.

Dr. Rick Sacra, a 51-year-old Boston physician, is the latest worker for SIM USA to be infected with Ebola and is receiving care in Liberia, according to the organization.

Sacra had volunteered to return to the country, where he has long offered medical services, when two other U.S. health workers became sickened with the virus during the most severe Ebola outbreak in history.

"I am ready to go," Sacra was quoted as saying by the president of SIM USA, Bruce Johnson, who spoke to reporters at the organization's headquarters in Charlotte.

Sacra had not been caring for Ebola patients but was delivering babies, the group said. It is not known how he contracted the disease.

The doctor was following protocols to prevent the disease, the organization said. It did not immediately provide updates on his condition, but said previously he was doing well.

Since March, more than 3,500 cases of the disease have been reported and more than 1,900 people have died in the West Africa outbreak, the World Health Organization said on Wednesday.

The hemorrhagic fever is only transmitted in humans by contact with the blood or bodily fluids of sick people, although suspected cases of airborne infection have been reported in monkeys in laboratories.

'VERY DARK DAYS'

While fear of infection has hampered international response efforts, it did not deter Nancy Writebol, another U.S. health missionary who contracted the disease in July, also while working at the missionary group's health facilities in Liberia.

"There were some very, very dark days," the 59-year-old mother from Charlotte told reporters, speaking at times through tears. "There were many times when I thought, 'I don’t think I am going to make it anymore.'"

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Appearing robust during a news conference at the SIM USA headquarters, Writebol said many factors helped save her life.

She was flown back to the United States to receive care in an isolation unit at Emory University Hospital in Atlanta, where she was treated along with Dr. Kent Brantly, another U.S. missionary who contracted the disease in Liberia.

Writebol and Brantly had worked together in the Ebola unit. They were among the few patients to receive an experimental treatment, ZMapp, although doctors at Emory said they could not determine whether it aided their recovery.

Brantly, affiliated with another missionary group called Samaritan's Purse, also survived the disease, which has an overall fatality rate of about 50 percent in the current outbreak, the WHO said.

"Was it those doctors and nurses that helped to save you, or was it your faith?" said Writebol, citing what many have asked her. "My answer to that question is all of the above."

(Writing by Letitia Stein; Editing by David Adams, Scott Malone and Peter Cooney)