An American doctor who was infected with Ebola while working with a medical missionary organization in Africa said in a statement that he is feeling "cautiously optimistic" as he fights the deadly virus. 

Dr. Peter Stafford was working with the missionary group Serge in the Democratic Republic of Congo when he was infected with the virus, the group said. He was evacuated to a hospital in Berlin, Germany, to receive care, the group said Tuesday.

"Before I was evacuated I was feeling really concerned I wasn't going to make it. And now I'm cautiously optimistic," Stafford said in a statement shared by Serge. 

Stafford's wife, Dr. Rebekah Stafford, who also works with Serge, and their four children were also evacuated to Germany, Serge said. They are asymptomatic and are being isolated and monitored, the group said Thursday. 

The Bundibugyo ebolavirus outbreak in Congo, which has spread to neighboring Uganda and South Sudan, is likely larger than what has officially been reported, health officials have warned. There are so far nearly 600 suspected cases, including 139 suspected deaths, the World Health Organization said.

Stafford was exposed to the virus while doing a surgery at Nyankunde Hospital in Bunia, a city in eastern Congo, Serge said in an earlier statement. He has worked at the hospital since 2023, according to the group. Ebolaviruses are transmitted from person to person through bodily fluids such as vomit, blood or semen, and medical personnel can be at high risk if exposed to sick patients. 

This is only the third known outbreak of the Bundibugyo strain. There is no known vaccine or treatment for it, health officials have said. The first symptoms are typically fever, fatigue, muscle pain, headache and a sore throat. Symptoms then can progress to vomiting, diarrhea, abdominal pain, rash, organ dysfunction and less frequently, internal or external bleeding. The fatality rate of the Bundibugyo strain is about 30 to 50%, according to the WHO.

Dr. Scott Myhre, the Serge director for East and Central Africa, described Stafford as "critically ill but not acutely deteriorating." Myhre said Stafford reported feeling better Thursday than the day before, and said that the infected doctor had been able to eat small amounts of food as he experiences Ebola symptoms including vomiting, rash and diarrhea. 

While there is no treatment for this type of ebolavirus, patients can receive supportive care including rehydration and specific symptom treatment. Stafford's medical labs are "trending slightly in the right direction," and he has received intravenous treatments "designed to improve Ebola outcomes," Myhre said. 

Hospital staff briefly allowed Stafford to see his wife and children through a hospital window, Serge said. 

Matt Allison, Serge's executive director, told CBS News on Tuesday that Stafford is "doing well, all things considered." 

"He's sick. He's sad to be away from his family, but he's getting the best care available to him," Allison said. 

A third Serge doctor, Dr. Patrick LaRochelle, was also potentially exposed while working in a DRC hospital. He is in quarantine at Bulovka Hospital in Prague, according to Serge. He remains asymptomatic so far, the organization said.

The State Department announced Thursday that any U.S. citizens or lawful permanent residents returning from the Congo, Uganda and South Sudan who had been in those countries within three weeks of entering the U.S. had to fly into Washington-Dulles International Airport in Virginia. The same restrictions apply to any non-citizens, according to the Department of Homeland Security.