Dengue fever is endemic in tropical and subtropical areas. Dengue fever is estimated by the World Health Organization (WHO) to cause about 50-100 million infections per year worldwide. The U.S. Centers for Disease Control and Prevention (CDC) considers dengue fever to cause the majority of acute febrile illnesses in travelers returning to the U.S. The first clinical report of dengue fever was in 1789 by B. Rush, although the Chinese may have described the disease associated with "flying insects" as early as 420 AD.