Dengue hemorrhagic fever is characterized by a sudden onset of fever that lasts for 2-7 days and then subsides, at which time hemorrhagic manifestations become evident. Sometimes there is an associated form of hypovolemic shock known as dengue shock syndrome. There are usually significant changes in the liver, the reticuloendothelial system, and the vascular system (e.g., necrosis of liver cells and focal hemorrhage, increase in turnover of lymphocytes, and diapedesis of erythrocytes through vessel walls). Because of the lack of pathologic findings in major organs and the rapid recovery (without sequelae) of survivors, physiologic dysfunction is thought to be secondary to the action of biologic mediators that are capable of producing severe illness with minimal structural injury.