Rates of syphilis and other sexually transmitted diseases (STDs) in Japan spiked in 2016. No one’s really sure why.

According to figures by the government-linked National Institute of Infectious Diseases (NIID), 4,077 people (link in Japanese) contracted syphilis as of Nov. 27 last year. Data from Japan’s Ministry of Health, Labour and Welfare show that 2,697 people (link in Japanese) had syphilis in the whole of 2015. The number in 2016 is about five times more than the 827 cases (link in Japanese) reported in 2011. According to the NIID, the number of syphilis cases in Japan has been steadily rising since 2010 (link in Japanese), but it does not have an explanation (link in Japanese) as to why.

Local media reports note that last year’s tally is on par with that in 1974 (link in Japanese), when 4,165 people contracted the disease. In 1967 the figure was about 11,000 (link in Japanese). Japan has been recording syphilis cases since the post-war period, when the disease was rife before the widespread introduction of antibiotics.

According to the Japan Times, syphilis is particularly rife in Shinjuku Ward, the center of Tokyo’s entertainment and nightlife. Syphilis rates there account for some 20% of the national rate, according to the paper.

The infection rate for women also rose quicker (link in Japanese) than for men in 2016. According to the data, of last year’s 4,077 reported cases, 2,848 were men and 1,229 were women, but the female rate increased 1.9 times from the previous year compared to 1.7 times for men. Women in their late 20s were particularly at risk.

In response, the Japanese government has signed up one of anime’s most popular female characters, Sailor Moon, to combat syphilis and other STDs. Sailor Moon was created by manga artist Naoko Takeuchi and was hugely popular in the 1990s. In a November press release, the Ministry of Health, Labour and Welfare said in a statement (pdf, link in Japanese) that it had teamed up with Takeuchi to launch a campaign to prevent STDs and syphilis in particular.

The campaign includes handing out condoms and pink leaflets printed with Sailor Moon’s face to passersby around Japan, as well as displaying posters educating people about the importance of getting tested. In one leaflet, Sailor Moon says, “If you do not get tested, you will be punished!!”—a nod to the character’s catchphrase, “In the name of the moon, I punish you!”

This article originally appeared on Quartz.