grade i infection is seen most frequently in world travelers, where it is usually self-limited and rarely fatal. The other grades are referred to as dengue haemorrhagic fever and are often fatal. Dengue haemorrhagic fever appears to be an infection by one of the other dengue viruses.
prior immunity to a different dengue virus type appears to be important in the development of the more serious haemorrhagic form. Vaccines are available. Protection from mosquitoes is an important preventive measure.