Dengue fever will become a major threat in the southern United States, southern Europe and new parts of Africa this decade, the WHO's chief scientist said, as warmer temperatures create the conditions for the mosquitoes carrying the infection to spread. “We need to talk much more proactively about dengue,” Jeremy Farrar, an infectious diseases specialist who joined the World Health Organization in May this year, told Reuters. Farrar previously spent 18 years working in Vietnam on tropical diseases including dengue.