A popular park in downtown Tokyo has been closed temporarily after dozens of cases of dengue fever were contracted by people who visited the area.
Health minister Yasuhisa Shiozaki on Friday urged the public not to panic, but said all local governments would be given guidelines on how to identify and handle dengue, which is spread only by mosquitoes.
"Nobody has died," Shiozaki said. "The public should remain calm."
Dengue is endemic in much of tropical Asia, infecting tens of thousands of people and killing hundreds each year in places like Malaysia and the Philippines.
Dozens of cases are brought back to Japan each year, but health officials recently confirmed the first locally transmitted cases in 65 years. As of Thursday, the number of infections was reported at 55.
All those listed as having contracted the disease locally said they had visited the park or nearby before they got sick, the health ministry says.
Symptoms of the disease include fever, severe joint pain and headaches. There is no treatment, and some of those infected can suffer from severe and life-threatening bleeding.
The outbreak has been top news in Tokyo.
Since the cases were discovered, workers have been spraying the heavily forested park, which is adjacent to the Meiji Shrine, a major tourist destination, and to Harajuku, a popular hangout for Japanese teens.
"I live nearby and have always ridden my bike and jogged in Yoyogi park. This is like my own garden. So I never expected this to happen," said Kiyoshi Takabayashi, 67.
This week, the U.S. Embassy issued a security message to Americans about the cases, urging caution.
Shiozaki said he was not expecting the outbreak to spread or the number of cases to rise significantly. But he urged medical facilities and health authorities to be vigilant.
"With global warming, we should keep in mind the increasing likelihood of things that were not seen in the past," he said.