Four different dengue viruses have been implicated in both dengue fever and dengue hemorrhagic fever. Dengue hemorrhagic fever occurs when the patient contracts a different dengue virus after previous infection(s) by another type. Prior immunity to a different dengue virus type is important in the production of this severe disease. Worldwide, more than 100 million cases of dengue fever occur every year. A small percent of these develop into dengue hemorrhagic fever. Most cases in the U.S. are brought in from other countries. It is possible for a traveler who has returned to the United States to pass the infection to someone who has not traveled. Risk factors for dengue hemorrhagic fever include having antibodies to dengue virus from prior infection and being younger than 12, female, or Caucasian.
| 
