According to provincial health experts, the frequency of elderly people older than the age of 60 contracting syphilis is going up, mostly because of risky sexual activities.

Zhang Xibao, director of the Guangzhou Institute of Dermatology, said the rate has been increasing by more than 10 percent annually since 2011.

"For people older than 70, the incidence is more than 50 for every 100,000 people. This is very high," he said. "These people are usually retired and lonely and it's not difficult to find cheap, but risky prostitution in the neighborhood."

The number of newly discovered HIV/AIDS cases among this small group of people is also increasing in the provincial capital of Guangzhou. Zhang said there is a possible link between the rise in syphilis cases and HIV/AIDS infections, Zhang said.

"Syphilis infection can break the mucous membrane of reproductive organs, which acts like a barrier to keep away sexually transmitted diseases, making it easier for people to get other STDs, including HIV/AIDS," he said. "Given that currently HIV/AIDS is mainly transmitted through sex, controlling other STDs means controlling HIV/AIDS."

He said the primary demographic of syphilis sufferers, however, is still people between the ages of 20 and 45. This group accounted for more than 50 percent of new syphilis cases in the province last year, mostly because they are more sexually active, he said.

Wang Xiaochun, director of hepatitis C and STD prevention for the National Center for AIDS/STD Control and Prevention, also stressed the importance of preventing and treating STDs in order to control the spread of HIV/AIDS.

"It's widely recognized that all STDs, including syphilis, are co-factors for HIV/AIDS because it's easier for people with STDs to get infected with HIV through sex," he said.

Syphilis and gonorrhea are classified as Category B infectious diseases because there is a high risk of an outbreak or they can easily be spread.

Wang admitted that a lot of people do not go to hospitals to diagnose or treat STDs.

"Many people, especially women, don't show any symptoms when first getting syphilis. Some even avoid hospitals and take medicines privately," he said.

"Syphilis is curable by early intervention, and the treatment costs much less than for other STDs. But I suspect that not many in the public know how it is transmitted, how to prevent it and where to seek treatment."

Most clinics in China that have voluntary counseling and testing for HIV/AIDS in China can perform a syphilis test for free.

By 2020, China has a plan to make more people, especially migrant workers, sex workers and men having sex with men, aware of syphilis. The province is also working to increase examinations of the STD.

Zheng Caixiong in Guangzhou contributed to this story.