European countries are seeing a growing number of locally transmitted tropical diseases, such as dengue fever and chikungunya, as climate change allows disease-carrying mosquitoes to expand their range and active season. The trend matters because infections once considered exotic risks confined to travel abroad are increasingly being caught without ever leaving Europe, raising the prospect that such diseases could eventually become permanently established there rather than appearing as isolated outbreaks.

The main driver is the Asian tiger mosquito (Aedes albopictus), which bites by day and night and has already established itself across southern Europe, with researchers expecting further spread northwards. A notable outbreak in Fano, Italy, saw 199 locally acquired dengue cases between mid-August and mid-October 2024, none linked to travel. Experts such as Riccardo Moretti of Italy's ENEA agency warn that warming conditions—already sufficient for the mosquito to survive even in England during summer—are extending the insects' active season in southern Europe, with adults now observed as late as December and as early as March, raising the risk of year-round populations and endemic transmission.