Viral hemorrhagic fevers (VHFs) are caused by a group of infectious diseases that damage the body's circulatory system, interfering with the blood's ability to clot. Bleeding occurs as a result of severe platelet dysfunction or thrombocytopenia. The group of viral agents that cause VHFs can affect multiple organ systems within the body. Hemorrhage and necrosis can occur in most organs, particularly the liver. Many of these viruses cause severe, life-threatening illness, while some others are much less threatening.