Dengue, also known as dengue fever, is a viral disease transmitted to humans by mosquitoes of the genus Aedes. The incidence of dengue is currently increasing dramatically, and it is now one of the diseases said to be re-emerging. WHO estimates the annual number of cases to be 50 million, including 500,000 cases of dengue hemorrhagic fever, which has a case fatality rate of over 20%. Two and a half billion people live in risk areas. Although initially seen only in the world's tropical and sub-tropical regions, dengue has now reached Europe, where the first two indigenous cases were reported on the French mainland in 2010. As of 2014, the mosquito vector is found in 18 French departments, and there is a real risk that the disease may spread if patients infected with dengue arrive in mainland France.