Paris (AFP) - Dengue fever, the world's fastest growing mosquito-borne disease, has spread its wings and morphed from a tropical disease endemic in just nine countries to worldwide threat.

Globalisation, urbanisation, climate change and jet travel have enabled it to move into more temperate zones.

Following are some basic facts:

- What is dengue fever ? -

Dengue fever is a flu-like infection caused by the flavivirus; it is in the same family as yellow fever.

Dengue fever has four separate strains -- DEN-1, DEN-2, DEN-3 and DEN-4. Once cured, patients are protected for life, but only against the strain they were stricken by.

- How is the virus transmitted ? -

Dengue is transmitted by several subspecies of the mosquito Aedes aegypti, which originate in Africa but which are now present in all tropical and subtropical areas.

- The symptoms -

Dengue can trigger a crippling fever along with headache, nausea, vomiting, muscle and joint pain, and skin rashes similar to measles.

- The most severe form -

The most severe form of the disease, dengue haemorrhagic fever, accounts for one percent of cases, killing 22,000 people a year, the World Health Organisation (WHO) says. It results in bleeding and blood plasma leakage. It can be particularly fatal among children.

- How many cases ? -

The number of dengue cases has risen 30-fold over the last 50 years, according to the WHO, making it the world's fastest-growing mosquito-borne disease and leaving more than half of the global population at risk.

The WHO says that half a million people are hospitalised by the illness every year, many of them children; of that number, roughly 2.5 percent die.

- Treatment and vaccination -

There is currently no specific treatment for dengue fever.

The world's first-ever dengue vaccine, Dengvaxia, manufactured by French pharmaceutical giant Sanofi, secured its first regulatory approval in Mexico on December 9.

The Philippines became the first Asian country on Tuesday December 22 to approve the sale of the vaccine.

Other pharmaceutical companies are developing dengue vaccines, including US firm Merck, Japan's Takeda and Britain's GlaxoSmithKline.