Dengue viruses may be introduced into areas by travelers who become infected while visiting tropical urban areas of the world where dengue commonly exists. Where Aedes aegypti (sometimes also Aedes albopictus) mosquitoes live, outbreaks of dengue occur. Dengue virus serotypes are now present in almost all the American Region. It was reported that DEN-3 was reintroduced into Central America in 1994 and is now found in several countries in the region. This serotype has been absent from the Americas for almost 20 years and because of this, the population has a low level of immunity and the virus is expected to spread rapidly.