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.