The main current culprit is the mosquito Aedes albopictus, commonly known as the Asian tiger mosquito (or just the tiger mosquito, given it’s now found across all continents). This is when tiger mosquitoes are most prevalent, and it’s also when more people return from travel. Someone unwittingly brings back an infection that isn’t found in Europe—dengue, say—and gets bitten by a tiger mosquito. Ultimately, says Moretti, there’s a risk that tropical diseases like dengue could become endemic in parts of Europe, meaning the disease is always there. Earlier this year, a study found that the conditions required for tiger mosquitoes to transmit chikungunya were cooler than previously thought.