Doctor talks with a patient infected with dengue virus at a hospital in Guangzhou, Guangdong province, Sept 28, 2014. [Photo/IC]

BEIJING - The dengue virus has killed six people and infected more than 20,000 in southern China's worst outbreak of the mosquito-transmitted disease in about two decades.

Authorities in worst-affected Guangdong province attribute the severity of this year's outbreak to exceptionally hot and wet weather, plus increasing travel by residents to dengue-affected regions in Southeast Asia.

The Guangdong health agency says on its website that 21,527 people had been infected by Sunday, with more than 1,000 new cases per day. Other provinces also reported a few dozen cases.

Dengue causes flu-like symptoms and rashes. It usually is not fatal, but can be especially severe for younger victims.

On Monday, the sixth day of China's week-long National Day holiday, reporters saw vehicles spraying pesticide in the streets of Guangzhou.

People were also disinfecting sewers, garages, channels and green belts in a bid to eradicate mosquitos. Residents could get free mosquito repellent at local communities.

As the holiday is drawing to an end, Guangzhou will launch a clean-up campaign on Tuesday, especially in schools and construction sites that remained unoccupied during the long holiday.