"He was never afraid of getting his hands dirty or his feet dirty," said Dr. Deogratias Niyizonkiza, founder of Village Health Works, the aid group that brought him to Burundi for four months in 2012.
"He went into this environment, a country that is truly off the mark, without knowing the language and he would make everyone feel so comfortable. It's really a daunting task and yet he helped the people immensely," Niyizonkiza said. "He talked to everyone, including the people working in the lab ... Their language was just to hug each other and smile."
In between it all, Spencer ran the ING New York City Marathon in 2013, finishing with a respectable amateur time of 3 hours, 43 minutes.
Spencer was hospitalized at New York City's Bellevue Hospital Center on Thursday, six days after returning from Guinea. Health officials said he began feeling tired on Tuesday, spent a day out in the city on Wednesday, and then alerted authorities when he developed a fever Thursday morning.
Experts have repeatedly assured the public that there is little chance that Spencer spread the virus prior to developing symptoms, but his case prompted the governors of New York and New Jersey on Friday to order a mandatory quarantine for any arriving international travelers who had contact with Ebola patients in three West African countries.
The first person to fall under the order was a healthcare worker returning Friday from treating Ebola patients in West Africa. By Friday evening, she had developed a fever but tested negative for Ebola in a preliminary evaluation, New Jersey officials said.
New York Gov. Andrew Cuomo criticized Spencer, saying he should have stayed home until any danger period for the disease had passed. The virus can hide in the body for up to 21 days before a person develops symptoms.
"Dr. Spencer is a valued fellow and was a volunteer and did great work, but that was a voluntary quarantine situation for 21 days. He's a doctor and even he didn't follow the voluntary quarantine, let's be honest," Cuomo said.
Neither the U.S. Centers for Disease Control nor Doctors without Borders ask healthcare workers returning from the Ebola hot zone to quarantine themselves, but they do recommend that they monitor their temperature at least twice a day. Spencer was complying with that guidance, officials have said.
Friends of the stricken doctor described him Friday as fun-loving but driven to stay involved in the global health fight.
"Everything else in his life was two, three and four on the priority list," said Dr. Liz Edelstein, a San Diego emergency medicine physician who met Spencer while teaching a course on wilderness medicine.
Doctors at Bellevue and city officials haven't released much information about his condition, but said he was well enough on midday Friday that he was speaking to people by cellphone.
Spencer's fiancee has also been quarantined at the hospital, though she had not developed any sign of the illness.
In an article that appeared in the journal of Emergency Physicians International last May, Spencer talked about some of the tough conditions he witnessed while working in Burundi. Those included a lack of basics as simple as aspirin, and having to ration a limited supply of oxygen for the gravely ill.
"In one case we gave oxygen to a child with pneumonia over a woman who was very anemic from malaria," he said. "The physicians in Burundi are some of the best I've ever come across ... I don't think I would have had the same success in deciding who needed oxygen to survive and who didn't. But they had experience. They'd seen it before."
Columbia University Mailman School of Public Health epidemiologist Leslie Roberts, who worked with Spencer on field research in Burundi, said in an email that he is "one of the most brilliant and delightful people" he'd ever worked with.
Spencer attended The Johns Hopkins University as an undergraduate, studied Chinese language and literature at Henan University, got his MD at Wayne State University School of Medicine, in Detroit, and his Masters in public health at Mailman, according to his LinkedIn page. His profile said he was also proficient in Spanish, French and Greek.