Japan is battling its first outbreak of dengue fever in almost 70 years, with at least 22 people confirmed as being infected.
The health ministry said the cases were believed to have been contracted by visitors to Tokyo's popular Yoyogi Park.
An outbreak of dengue fever was last recorded in Japan in 1945.
The tropical disease, which is spread by mosquitoes, causes symptoms like high fever and severe joint pain.
Severe cases may require hospital treatment and can occasionally develop into a potentially lethal complication called severe dengue, according to the World Health Organisation.
Japan sees imported cases of dengue fever each year, mostly from tourists who catch it while travelling to tropical regions, its health ministry said.
Domestic mosquitoes could have also picked up the dengue virus from tourists and passed it on, the ministry said.
Now the government is working to tackle the park thought to be the focus of the outbreak.
New warning signs have been put up and teams of workers in white overalls and masks have descended to spray insecticide and drain its ornamental ponds, says the BBC's Rupert Wingfield-Hayes in Tokyo.
The spread of the disease has been helped by an exceptionally wet summer but authorities say the outbreak should be killed off by the onset of autumn weather, our correspondent adds.
There are no vaccines or drugs against dengue but medical experts said avoiding mosquito bites was the best precaution.
A government hotline for dengue fever queries was also set up last week.
The disease is common in more than 100 countries across Asia, Africa and Latin America.