Shedding a rain-spattered windbreaker, the lanky Bostonian hurries to join the morning rounds at McCord Hospital, an oasis of health care in this largest city of KwaZulu-Natal province—a region at the epicenter of South Africa's HIV/AIDS epidemic. As chart-wielding doctors and nurses move from patient to patient in a 25-bed men's ward, Walker hears a weak voice call out. He leans over the bed's paint-chipped railing to listen to the patient, a 17-year-old Zulu, describe his symptoms.

Photos: Paul Weinberg