Two decades ago HIV was ripping across Africa like flames across a lake of petrol. In some countries more than a quarter of adults were infected. Nearly all were expected to die, slowly, leaving families without breadwinners and forcing girls to drop out of school to care for sick parents. Sober observers predicted social collapse. But then the price of antiretroviral drugs plunged: pills that not only kept people alive but made them less infectious. By a conservative estimate, they saved 21m lives.