Flu season has arrived -- six weeks earlier than last winter and its second-earliest onset in the past decade, the health ministry announced Tuesday.

The announcement is based on a survey of some 5,000 pediatric and internal medicine hospitals and clinics nationwide. Flu season is defined as arriving when more than one patient per institution has the flu.

Preliminary data for Dec. 12-18 showed a total of 8,822 flu patients, or an average of 1.88 per institution, topping the 1.00 threshold for the first time this season, according to weekly surveys by the ministry and the National Institute of Infectious Diseases.

The earliest arrival of flu season in the past decade was the winter of 1996-1997, according to the Health, Welfare and Labor Ministry.

The ministry advised people to wear face masks in public, and to wash their hands and gargle after being out.

Viruses detected so far have been mostly type A with a few cases of type B, which was predominant last year. The type-A strain can cause more severe symptoms.

Outbreaks at schools began in November, with around 5,200 cases reported at kindergartens, elementary schools and junior high schools between early November and mid-December, the ministry said.