Hard-working shopkeepers could not get out of bed because their hips hurt so much. Able-bodied men could not lift spoons to their mouths. (Months later, many still have debilitating joint pain.)

From the start, doctors suspected that the disease was spread by insects, rather than people. While almost all homes had one person who was ill, family members seemed not to catch the disease from one another.

But the biggest mystery is whether chikungunya will emerge here next summer. In the tropics, it is a year-round disease, since the mosquitoes breed continually. But the virus can winter over in mosquito eggs, too, and no one knows if there are reservoirs of sleeping eggs in some pool of water in Italy.

With climate change at hand, Bertollini said, chikungunya will surely be back somewhere in Europe again.