Rick Sacra, a doctor from the Boston area, was identified Wednesday as the fourth American to be infected with the Ebola virus. Sacra reportedly contracted the deadly disease while working at a hospital in Liberia.

Bruce Johnson, head of the missionary group Serving in Mission, or SIM USA, which sponsored Sacra's work in the West African country, said that it was not known exactly how Sacra contracted the deadly virus, and added that all of Sacra’s patients in Monrovia were being screened for Ebola, according to a report in the New York Times. Sacra, 51, was reportedly working in a maternity ward of a hospital in Monrovia.

“A strong possibility is that the Ebola symptoms were masked and not presenting themselves with a patient who was admitted and cared for,” Johnson reportedly said.

Sacra is currently being treated in the Ebola ward at the Eternal Love Winning Africa, or ELWA, hospital run by SIM in Monrovia, the same hospital where he reportedly contracted the disease. Johnson said there were no plans to bring the infected doctor back to the U.S., and added that Sacra had not received the experimental Ebola drug ZMapp.

Sacra, who had previously worked in Liberia, reportedly decided to go back to the country after two other American missionary workers, Kent Brantly and Nancy Writebol, contracted the deadly virus.

“He approached us after hearing of Dr. Brantly being unable to carry on his medical duties,” Johnson reportedly said, adding: “Plus, our other two doctors had been pulling very long hours, under extremely fatiguing conditions, and they needed a break.”

Brantly and Writebol, the American aid workers who contracted Ebola in West Africa, were flown to the U.S. and treated at an isolation unit in Atlanta’s Emory University Hospital and given the experiment drug ZMapp. Both Writebol and Brantly were discharged from the hospital late last month.

Writebol, 59, making her first public appearance Wednesday since being discharged, recounted her personal experience battling the disease. Writebol reportedly spent the last few days in seclusion with her husband, and said she did not know whether to credit the experimental drug ZMapp, or the care she received, for her recovery but said that she believed God played an important role, the Times reported.

The third American patient, a Liberian-American, who contracted Ebola after he arrived in Nigeria, died in the African country soon after being infected with the disease.

According to figures released by the World Health Organization, or WHO, on Wednesday, more than 1,900 people have died and over 3,500 have been infected from the largest outbreak of Ebola that has ravaged the West African countries of Liberia, Guinea and Sierra Leone in the last few months. Ebola cases have also been confirmed in neighboring countries like Nigeria and DR Congo as well.

WHO fears that more than 20,000 people could get infected by the Ebola virus, which was first identified in 1976, before the current outbreak runs its course. The second major outbreak of the disease happened in 1995, also in West Africa. Ebola outbreaks have a fatality rate of up to 90 percent, according to the WHO.