Hemorrhagic fevers are caused by viruses that exist throughout the world but are most common in tropical areas. Early symptoms, such as muscle aches and fever, can progress either to mild illness or to a more serious, potentially fatal disease. In severe cases, a prominent symptom is bleeding (hemorrhaging) from orifices, such as the nose or vagina, and from internal organs.

These viruses also exist in insects, spiders and other animals that become infected but do not die and often have no symptoms. The viruses are transmitted to humans either directly by one of these infected animals or by an intervening species such as mosquitoes.

Hemorrhagic fevers are generally linked to specific locations. If many people reside in one of these areas, the number of cases may be high. Fatality rates are variable. In cases of dengue hemorrhagic fever-dengue shock syndrome, 1-5% of those afflicted die...