In 1952, the Chinese government launched an unprecedented campaign, instituting mass screening for the T. pallidum germ, providing free treatment to infected individuals and closing brothels. By the 1960s, the initiative virtually eradicated syphilis in China.

The Lancet paper says that this success ironically worsened the danger for the Chinese population when the country opened up its economy in the 1990s, unleashing the social earthquake that continues to this day.

As syphilis had been virtually absent for 20 years, the general population of young, sexually active individuals had become "completely susceptible" to infection, it says.

The driver for the epidemic has mainly been sex work, which has risen with the expansion of China's vast, shifting population of migrant workers.

There have also been changes in sexual habits, including a move towards sexual intercourse at an earlier age, with more partners and before marriage but also with poor use of condoms.

Syphilis prevalence is highest in the big-growth regions of coastal China, led by Shanghai (55.3 cases per 100,000), Zhejiang (35.9) and Fujian (26.8).

This was followed by Beijing (24.9 cases per 100,000) and the Zhujiang river delta, comprising Guangxi, Guangdong and Hainan provinces, with rates of 14-21 per 100,000 individuals.

Other countries have likewise reported a resurgence of syphilis in high-risk groups recent years. The United States reported a 2.7 per cent nfection rate in 2004.

The paper, lead-authored by Chen Xiangsheng, admits that the picture could be even worse, as the data is based on 26 nationwide "sentinel sites" which receive details of patient admissions from government STD clinics.

Many people, though, may get treated at family planning centres, gynaecological clinics and other facilities or by pharmacists or private practitioners, and these cases go largely unreported.

In addition, the increasing privatisation of health care in China has left many people without the resources to get screened or treated for syphilis.

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