French health authorities are warning of the risks of imported cases of dengue fever ahead of the Olympics due to the ongoing surge of the virus in the Americas. According to the World Health Organization (WHO), there have been more than 5.2 million cases of dengue in the Americas, an increase of more than 400 percent compared to the average over the past five years. The virus, which is transmitted to humans through the bite of an infected mosquito, can be severe or fatal in some cases. French health authorities have reported more than 1,649 imported cases of dengue this year, mostly from French overseas territories of Guadeloupe and Martinique. They are calling on health professionals and the public to be vigilant, especially as the tiger mosquito that transmits the dengue virus has established itself in Europe. When traveling to regions with high levels of dengue, people should wear loose clothing that covers the skin and use insect repellent and mosquito nets to prevent bites.