Japan reported in 2017 over 5,000 syphilis patients for the first time since comparable data became available in 1999, with the number rising particularly among women in their 20s, data by a national research center showed Friday.

With Tokyo accounting for roughly 30% of the reported patients, the metropolitan government is set to start providing more free syphilis tests from April to contain the disease, which is curable at an early stage but has serious consequences if left untreated.

As of Dec 17, 2017, the total number of syphilis patients in Japan stood at 5,534, up from 4,518 in 2016, according to the National Institute of Infectious Diseases. The cause behind the increase in the number of people contracting the sexually transmitted disease remains unclear.

By area, Tokyo reported the largest number at 1,705, followed by Osaka Prefecture at 788, Aichi Prefecture at 325, and Kanagawa Prefecture at 312.

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