FACTBOX-The when, where, why and how of Asia's deadly dengue

Oct 17 (Reuters) - Dengue, a mosquito-transmitted disease which causes fever, headaches and agonising pain in muscles and joints, has killed 389 people and infected more than 38,000 in Cambodia, which is battling one of the worst outbreaks in years.

Here are some facts about dengue, which poses a threat to 2.5 billion people, or two-fifths of the world's population, according to the World Health Organisation (WHO).

WHAT:

-- A severe, flu-like illness, caused by four distinct, but closely related, viruses. Known colloquially as bone-break fever for the severe muscle and joint pains it induces, the disease causes death only when it develops into a more severe form, dengue haemorrhagic fever (DHF).

HOW:

-- Virus-carrying female Aedes mosquitoes infect humans with the disease when they bite. It takes 4-7 days before a person shows symptoms of the disease.

-- After incubating the virus for 8-10 days, the black and white striped mosquitoes can transmit it to susceptible individuals for the rest of their two-week life span.

WHEN:

-- Dawn and dusk are peak biting times for the silent, aggressive Aedes, whose name comes from the Greek for "unpleasant". Dengue can peak during rainy seasons when mosquito eggs hatch in stagnant water.

WHERE:

-- Tropical and sub-tropical regions. Southeast Asia and the Western Pacific are most seriously affected.

-- More common in urban areas where Aedes like to live, the disease is endemic in more than 100 countries in Africa, the Americas, the Eastern Mediterranean, Southeast Asia and the Western Pacific.

HOSPITALISATION:

-- The WHO says there may be 50 million cases of dengue worldwide each year of which 500,000 are treated in hospital for DHF, many of them children. At least 2.5 percent of hospital cases die annually.

TREATMENT:

-- With no specific treatment for the four viruses, health officials focus on eradicating mosquitoes to prevent infections.

Sources: Reuters, The World Health Organisation (www.who.int/mediacentre/factsheets/fs117/en/)