Data from the National Institute of Infectious Diseases showed Friday that in 2017, for the first time since comparable data became available in 1999, there were over 5,000 syphilis patients domestically, with the number rising particularly among women in their 20s.

According to the national research center, as of Dec. 17, 2017, the total number of syphilis patients stood at 5,534, up from 4,518 in 2016. The cause of the increase in people contracting the sexually transmitted disease remains unclear.

With Tokyo accounting for roughly 30 percent (1,705 cases) of reported patients, in April the metropolitan government will start providing more free syphilis testing to contain the disease, which is curable at an early stage but has serious consequences if left untreated.

To prevent further spread of the disease ahead of the Tokyo Olympic Games in 2020, the metropolitan government has earmarked funds in its fiscal 2018 budget to conduct more free anonymous tests and train doctors who have limited experience in treating syphilis.

Syphilis is caused by bacteria called treponema. In the primary stage, the disease manifests as minor tumors in affected areas, such as the genital area and lips, and can be cured by anti-bacterial drugs. But if left untreated, the infection can eventually cause inflammation throughout the body as well as brain and heart complications.

Pregnant women who contracted the disease before or during pregnancy can pass it to their baby. The disease, known as congenital syphilis, can cause stillbirth or affect infants through symptoms such as meningitis and skin rashes.

After Tokyo, Osaka Prefecture had the highest number of cases at 788, followed by Aichi Prefecture at 325 and Kanagawa Prefecture at 312.