Panic was spreading this August through this tidy village of 2,000 as one person after another fell ill with weeks of high fever, exhaustion and excruciating bone pain, just as most of Italy was enjoying Ferragosto, its most important summer holiday.

“At one point, I simply couldn’t stand up to get out of the car,” said Antonio Ciano, 62, an elegant retiree in a pashmina scarf and trendy blue glasses. “I fell. I thought, OK, my time is up. I’m going to die. It was really that dramatic.”

By midmonth, more than 100 people had come down with the same malady. Although the worst symptoms dissipated after a couple of weeks, no doctor could figure out what was wrong.

People blamed pollution in the river. They denounced the government. But most of all they blamed recent immigrants from tropical Africa for bringing the pestilence to their sleepy settlement of pastel stucco homes.

“Why immigrants?” asked Rina Ventura, who owns a shop selling shoes and purses. “I kept thinking of these terrible diseases that you see on TV, like malaria. We were terrified. There was no name and no treatment.”

Oddly, the villagers were both right and wrong. After a month of investigation, Italian public health officials discovered that the people of Castiglione di Cervia were, in fact, suffering from a tropical disease, chikungunya, a relative of dengue fever normally found in the Indian Ocean region. But the immigrants spreading the disease were not humans but insects: tiger mosquitoes, who can thrive in a warming Europe.

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