Viral hemorrhagic fevers (VHFs) are diseases caused by groups of viruses such as the Ebola virus and Marburg virus. In general, infection with these viruses may cause serious illness with fever and damage to blood vessels affecting many organ systems. Damage to the blood vessels can disrupt blood flow and cause severe bleeding. Although some VHFs are mild illnesses, many can be fatal. Other VHFs include Lassa fever, yellow fever, Crimean-Congo and New World hemorrhagic fever viruses.