The number of influenza patients is rising sharply nationwide, up an estimated 200,000 in the week from January 6 to 12, according to statistics compiled by the Health, Labor and Welfare Ministry.
There is a growing number of infections with a different type of virus than those prevalent in the previous two flu seasons.
The ministry has advised people to take thorough preventative measures, since the infection may spread to infants who lack immunity to the virus.
According to the ministry, the number of patients newly reported by 5,000 medical institutions established as observation points nationwide reached 27,100 in the week starting Jan. 6.
This is 2.7 times more than in the preceding week.
Based on this figure, the number of patients across the country is estimated to have reached about 340,000, up 200,000 in the week.
An increase in patients has been observed in all prefectures.
The flu was initially prevalent in western Japan, with Okinawa Prefecture seeing 19.90 patients per medical institution, Kagoshima 9.28 patients, Gifu 8.91 and Kochi 8.71.
However, there has recently been a marked increase in the metropolitan area, with Tokyo at 5.07 patients per institution, and the prefectures of Kanagawa at 5.1 patients, Saitama at 6.29 patients and Chiba at 5.7 patients. The spread is expected to peak early next month, as it started at the end of December, which was later than usual.
Research by the National Institute of Infectious Diseases indicates that the majority of infection cases are a type A Hong Kong virus, the kind prevalent in last year's flu season.
However, cases of infection with H1N1-type viruses, which emerged as a new type of influenza in 2009, are on the rise.
This type of virus accounts for almost 30 per cent of infections this season, compared to just 2 per cent from 2012 to 2013, and 0.2 per cent from 2011 to 2012.
Antiflu medicine such as Tamiflu and Rapiacta have proven ineffective against H1N1-type viruses.
Types that are resistant to these medicines have been confirmed in Sapporo.
The institute detected nearly the same type of drug-resistant virus in two adult patients and four patients under 10 years old from mid-November to the end of December.
Six patients were not given antiflu medicine.
"There is a high possibility that the same type of drug-resistant viruses are spreading," an institute official warned.
The ministry is advising doctors and other medical workers to use other antiflu medicine such as Relenza and Inavir, which have proven effective against H1N1 viruses.