When a 59-year-old man developed a fever and stomach ache on the French Mediterranean island of Corsica last August, his diagnosis surprised local doctors and sent a warning signal to the wider medical world. He had contracted malaria.

"Patient Y" fitted none of the usual explanations for the small number of cases that occurs each year in Europe, where the disease was eradicated decades ago. He had never travelled to a country where the parasite was endemic, nor for 10 years had he even been inside an airport to which an infected mosquito might have been inadvertently transported by aeroplane.