Dengue fever is a viral illness spread by mosquitoes. There are four closely related dengue viruses (DENV 1, DENV 2, DENV 3, and DENV 4). These viruses are transmitted to humans by the bite of an infected mosquito. The disease occurs mainly in tropical Asia, Africa, the Caribbean, and the South Pacific. It is most common during the rainy season in areas infested with infected mosquitoes. Sometimes, persons arriving from other countries may enter the United States with dengue fever and infect local mosquitoes, as happened in Hawaii in 2001 and 2011. The current 2015 Hawaii Island outbreak involves DENV serotype 1.