Dengue fever a very old disease has re-emerged in the past 20 years with an expanded geographic distribution of the viruses and the mosquito vectors increased epidemic activity the development of hyperendemicity and the co-circulation of multiple serotypes. In 1998 this mosquito-borne disease was the most important tropical infectious disease after malaria with an estimated 100 million cases of dengue fever 500000 cases of dengue hemorrhagic fever and 25000 deaths annually in the tropics. The reasons for this resurgence and emergence of dengue hemorrhagic fever in the waning years of the 20th century are complex and not fully understood but