One recent morning, three figures in white lab coats descended cautiously into a pitch-black netherworld beneath a crumbling bridge in the Afghan capital. They picked their way through garbage and sprawled limbs, passing hundreds of huddled men whose gaunt, wary faces were briefly illuminated by the flare of matches and drug pipes.

The doctors were headed to a lone tent pitched nearby on the dry riverbed, where they knew that a female addict named Marzia had been sleeping on her own. They approached quietly, saying they had come to help. From within came shouts of “Go away, leave me alone!” Suddenly the young woman flung open the tent flap, cursing and hurling debris. Stumbling along the riverbed, she darted under the bridge and vanished into the protective company of fellow lost souls.