Dengue fever
Thomas Milton Rivers published the first clear description of a virus as distinct from a bacterium in 1927. The discovery of the West Nile virus came in 1937, and has since been found in "Culex" populations causing epidemics throughout Africa, the Middle East, and Europe. The virus was introduced into the Western Hemisphere in 1999, sparking a series of epidemics. During the latter half of the 20th century, Dengue fever reemerged as a global disease, with the virus spreading geographically due to urbanization, population growth, increased international travel, and global warming, and continues to cause at least 50 million infections per year, making Dengue fever the most common and clinically important arboviral disease.