Wu said it is impossible to impart knowledge about HIV/AIDS to all of China's migrant workers.

"With limited funds, we should seek to spread practical information to high-risk groups, such as the use of condoms," she said.

"We have been paying close attention to the floating population since 2000, when we first found the group was more vulnerable to HIV infection," said Xu Peng, a researcher at the Chinese Center for Disease Control and Prevention.

In China, the floating population refers to those who work outside their household registrations, or hukou.

Xu said that although some members of the floating population are well-educated white-collar workers, the vast majority of them are migrants who work in plants or as construction workers.

"Construction workers tend to resort to prostitution, because they cannot afford to have their wives live with them in cities," Xu said.

According to statistics from UNAIDS, 20 percent of male migrant workers said they had visited prostitutes and almost 70 percent of new HIV infections in 2009 were transmitted through sexual contact.

"The vulnerable groups include prostitutes, gay people, drug addicts and migrant workers. Among them, migrant workers are the largest group," said Chen Qingfeng, head of the policy coordination department of the Office of the State Council Working Committee for AIDS Prevention and Control.

The National Population and Family Planning Commission statistics from 2006 showed that 84 percent of migrant workers had no idea about how HIV was transmitted.