He traveled to Rwanda, Burundi and the Democratic Republic of Congo, among other places. Dr. Elizabeth Edelstein, an emergency physician in San Diego, knew him from New York and often invited him to lecture at wilderness medicine seminars she led. Sometimes he did, but other times, he demurred. “He’d love to come out and teach, but he couldn’t make the commitment because he might be in Africa,” she said.

In September, he traveled with Doctors Without Borders to an Ebola treatment center in Guéckédou, Guinea, a dusty town of muddy roads and open-air markets still bearing the scars of the civil war in Sierra Leone in the 1990s.

He worked as a doctor, seeing patients, and as an epidemiologist, a colleague at Doctors Without Borders said on the condition of anonymity because the group told its doctors not to speak on the matter. There are 90 beds at Guéckédou, including 15 for suspected cases. The treatment center is on the grounds of the main government hospital in the center of town, on top of a hill, a collection of worn buildings and the now-familiar sight of tents alongside them.

The center, largely empty of patients as recently as July, is now full.

He boarded a plane to leave Guinea on Oct. 14. A friend joked on Facebook: “Watch out for the Ebola!” But with the jokes came deep concern for Dr. Spencer’s safety. “We can honestly say we’ll be so happy when Craig returns to the States,” his mother, Pat Casey-Spencer, wrote on Facebook, according to Mr. Cedar. “We admire compassion to help the sick but we’ve never wanted him home more so now than ever.” She called Ebola “so devastating.”

He arrived at Kennedy International Airport on Oct. 17. His every movement since shortly after that jet touched down is now facing intense scrutiny.