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ROME — Malaria killed a 4-year-old girl in Italy, raising fears that the mosquito-borne disease has returned to the country after decades.

Sonia Zago developed a high fever after returning home from a vacation in Bibione, a seaside town on the Adriatic coast near Venice.

As her condition worsened, she was taken to a hospital in the northern city of Trento. Within an hour of arriving, the girl had fallen into a coma and was diagnosed with malaria. Zago was then transferred to a hospital in Brescia that treats tropical diseases, but she died on Sunday night.

The girl had never traveled to a risk-prone country, raising questions about how she contracted the disease.

"It baffles us how she could have been infected"

The Ministry of Health confirmed Tuesday that it had dispatched a team of experts to investigate.

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Dr. Claudio Paternoster, director of the infectious diseases ward at Santa Chiara hospital in Trento, said that he had not seen a case of home-grown malaria during his 30-year career.

He said that Zago had been admitted to the facility about two weeks ago due to suspected diabetes and had shared a ward with two children who had contracted malaria while in Burkina Faso.

"It’s not probable, almost impossible, to pass on the [parasite] from patient to patient," Paternoster told NBC News. "So the only option I can think of is that the mosquito that carried the disease may have traveled to Italy, and survived somehow."

Tropical diseases expert Alberto Matteelli, left, is flanked by Ezio Belleri, general manager of the "Spedali Civili" hospital, as they meet the media in Brescia, Italy on Sept. 5, 2017. Italy's health ministry has ordered an investigation into the death of a 4-year-old girl from malaria after checks determined she hadn't traveled to any country at risk for the disease. Filippo Venezia / ANSA via AP

He added: "It baffles us how she could have been infected.”

Italy has been malaria-free since the 1950s, with most recorded cases linked to tourists who returned from countries where it is common.

Paternoster speculated that climate change may be to blame.

”It was a long and very hot summer," Italian daily Corriere della Sera quoted Paternoster as saying.

On Tuesday, the World Health Organization was hosting a meeting in Moscow to discuss how to keep Europe malaria-free. Zero cases of home-grown malaria were reported in Europe in 2015.

The WHO says Italy could be vulnerable to a return of malaria if mosquitoes are not properly controlled.

Malaria is caused by a parasite that has a complex life cycle dependent on both animals, including humans, and mosquitoes.

CORRECTION (Sept. 5, 2017, 2:45 p.m. ET): Due to a mistranslation in a quotation from Dr. Paternoster, a previous version of this article incorrectly described the nature of malaria. It is a parasite, not a virus.