On Sept. 30, 2023, an anxious father brought his 5-year-old son to the hospital in Kamituga, a muddy, bustling town carved out of the thick forest in the eastern Democratic Republic of Congo. The boy had a high fever and oozing sores on his torso and face.

Nurses diagnosed chickenpox. They admitted him to the pediatric ward, part of a sprawl of low-slung brick buildings that date to the colonial era, and tried to manage his fever.

Days passed, and the child’s health did not improve. His fever climbed higher, the lesions spread, blistering even the soles of his small feet.

Perplexed, the pediatric staff called Dr. Steeve Bilembo, who was managing urgent care. He and a trusted nurse colleague, Fidèle Kakemenge, examined the boy and named, and then quickly eliminated, possibilities: Not chickenpox, not measles, not rubella, not a bad case of dermatitis — he would be improving by now if he had any of those ailments.

The spreading sores meant it wasn’t malaria or typhoid or cholera, the diseases afflicting other children in the crowded ward.

“And then at one point, we said, ‘Could it be mpox?’” Dr. Bilembo recounted. “Although we have never seen it — only in books.”