Public health authorities are worried about a confluence of events set to happen soon: Brazil is suffering the world’s worst epidemic of dengue fever, with 1.4 million cases last year. And the country is about to play host to the World Cup, which attracts tourists by the millions.

Officials fear some will carry the virus back to their home countries, where it can be spread if they are bitten by mosquitoes from the genus Aedes.

The tournament lasts from June 12 to July 13 in a dozen cities scattered around Brazil.

A team of Brazilian, Spanish and British scientists analyzed 13 years of climate data, looking for mosquito-friendly conditions.

Their study, published last week by Lancet Infectious Diseases, concluded that the threat would be greatest in Natal, Fortaleza and Recife, which are all on the northeastern coast.