After years of denial about the extent of AIDS in the world’s most populous country, China’s leaders have unleashed a well-financed campaign to stem an epidemic that by the government’s reckoning afflicts 840,000 people. But these efforts are being undermined by a relentless drive for profit within the Chinese health care system, a product of the country’s shift toward capitalism. Doctors at local hospitals responsible for dispensing free pharmaceuticals are exploiting those in need, padding bills with unneeded drugs and dubious services, according to medical experts, government officials and patients.