The flaviviruses (Latin flavus meaning yellow, because of the jaundice induced by yellow fever virus) comprise a large genus of medically important, arthropod-transmitted, enveloped viruses with members that include yellow fever, dengue, West Nile, tick-borne encephalitis (TBEV), and Japanese encephalitis viruses Burke and Monath 2001 and Lindenbach and Rice 2001. Dengue virus is one of the most significant human viral pathogens transmitted by mosquitoes and causes 50 million or more cases of infection worldwide each year, resulting in around 24,000 deaths (World Health Organization, 1998). Infection is usually characterized by fever and severe joint pain, but more serious syndromes, dengue hemorrhagic fever or dengue shock syndrome, occur sometimes following dengue infection. Dengue hemorrhagic fever was mostly confined to Southeast Asia until the 1960's, when it also became endemic in Central America and, more recently, in South America. There are four distinct serotypes of dengue virus, and it has been postulated that hemorrhagic fever, or shock syndrome, is usually the result of sequential infection with multiple serotypes. Although vaccines have been developed for several flaviviruses, control of dengue virus through the use of vaccination has proved to be elusive (Burke and Monath, 2001).