The Americas, South-East Asia and the Western Pacific are most seriously affected. Some 2.5 billion people - two-fifths of the world's population - are now at risk for acquiring dengue.
A rapid rise in unplanned urbanization is bringing ever greater numbers of people into contact with Ae. aegypti mosquitos by increasing favourable breeding sites for the mosquitos. These include peri-urban and slum areas where household water storage is common and where solid waste disposal services are inadequate.
Infection with one serotype predisposes individuals to DHF when subsequently infected with a different serotype. DHF now occurs regularly in countries that previously reported only DF because of the introduction and circulation of multiple dengue virus serotypes.
Without proper treatment, DHF case fatality rates can exceed 20%. With supportive therapy, it can be reduced to less than 1%.