For over two hundred years, dengue fever was seen as a benign fever suffered by visitors to the tropics. Back then, the disease was confined to relatively small geographic regions and the four different types of viruses remained isolated. Dengue outbreaks did occur, yet rarely progressed to the more deadly DHF.
 

Today, as the number of epidemics caused by multiple virus types grows, each succeeding outbreak becomes more vicious. Before 1970, only nine countries had experienced DHF epidemics. By 1995, the number had increased by four-fold.
 

There is a small but significant risk of dengue outbreaks in the United States. Each year, people returning from tropical areas where dengue is endemic import the viruses. Southern Texas and the southeast US states where A. aegypti is found are at risk of a dengue fever outbreak.