China has seen a dramatic rise in cases of syphilis, as a result of rapid social change, US researchers say.
A report in the New England Journal of Medicine says syphilis is now the most commonly reported communicable disease in Shanghai, the second-largest city.
Pregnant women are also increasingly passing the disease to their children, with more than one baby with congenital syphilis born every hour in 2008.
No other country has seen such a rise since the discovery of penicillin.
Syphilis was almost eradicated from China in the 1950s with a sweeping programme of antibiotics and brothel shutdowns.
But the researchers say the disease is now "a major scourge lurking in the shadows", with around 20 people in every 100,000 in China carrying the infection.
They link the rise directly to China's economic reforms and the growing number of migrant workers and men with expendable income, which has led to a growth in the commercial sex industry.
Female sex workers and gay or bisexual men - a third of whom are married - "disproportionately bear the burden" of the epidemic, they say.
Syphilis, a bacterial disease, is curable with antibiotics if treated early, but can cause paralysis, blindness and death if left untreated.
Mothers can pass syphilis to unborn babies, which can lead to deformities, neurological problems, stillbirths or death in early infancy.
The authors say there is a lack of adequate screening in China, and entrenched social stigma makes people reluctant to get tested.
They recognised China's efforts to increase awareness of sexually transmitted infections among sex workers, but say more needs to be done, including free screening for all pregnant women and high-risk groups.
Offering tests in non-clinical sites - including brothels, saunas and nightclubs - would be one way to help identify and prevent the spread of local infections, they say.