Clinically, the combination of fever and hemorrhage can be caused by many pathogens, including viruses, rickettsiae, bacteria, protozoans, and fungi. However, the term viral hemorrhagic fever (VHF) is usually reserved for systemic infections characterized by fever and hemorrhage caused by a special group of viruses that belong to four different families: Filoviridae, Arenaviridae, Bunyaviridae, and Flaviviridae. All members are lipid-enveloped RNA viruses; except for filoviruses, where the resorvoir remains unknown, these viruses are transmitted to humans by arthropods and rodents. The VHFs are characterized by abnormal vascular regulation and damage. The VHFs all have similar clinical pictures with mortality rates of 15-30% or in the case of Ebola virus, up to 80%. Between 1993 and 2001, more different VHFs and related diseases were reported than during any comparable period over the past several decades. These diseases include Ebola VHF in Uganda in 2000, in Zaire in 1995 and in Gabon between 1994 and 1996; ongoing cases of Marburg hemorrhagic fever in the Democratic Republic of Congo beginning in 1998; Rift Valley fever in Saudi Arabia and Yemen in 2000 and in Kenya in 1997; Lassa fever in Sierra Leone in 1997; hantavirus pulmonary syndrome in the Americas since 1993; and imported cases of dengue hemorrhagic fever from Central and South America, and yellow fever from Africa during the 1990s.