On a morning in early April, Geoffrey Chanda's phone was going off almost constantly. Truck drivers were calling him. "They are crying: 'We've got no [HIV] medicine. Where do you get [it] from?' " says Chanda, 54. For 15 years, Chanda has been meeting truckers in dusty parking lots at the border of Zambia and the Democratic Republic of Congo to give them their HIV medications. Now, he says, he doesn't know what to tell them. He's lost his job as a community health worker. The U.S.-funded program he worked for — which supported the mobile clinic where he collected...