Flu cases have more than doubled in the past week in England and Wales, according to new figures.
The biggest increase is in school age children, data from GP surgeries shows.
Several flu strains are known to be circulating widely, including H1N1 "swine flu" and influenza B.
Experts are urging those in high-risk groups, particularly pregnant women, to get vaccinated. The vaccine protects against both seasonal flu and swine flu.
The figures come from The Royal College of GPs, which tracks cases of flu-like illnesses at about 70 surgeries in England and Wales.
Cases have risen to 87.1 per 100,000 people, from 32.8 in the previous week.
This falls far short of the number of people catching flu at the height of last summer's swine flu outbreak.
But it is higher than seasonal flu outbreaks of the past few years.
"There's a fair bit of flu around but we're not overwhelmed by it," Dr Maureen Baker, Health Protection Lead at the Royal College of GPs, told the BBC.
Rates of flu are highest in children aged between 5 and 14, followed by children under four, then those aged between 15 and 44.
Cases have also increased in adults aged between 45 and 64, says the weekly bulletin from the Royal College of GPs.
The Central region (including Wales) has the most cases, followed closely by the South, with fewer cases in the North.
It is not clear how many of these patients have H1N1 flu, influenza B and other strains.
On Tuesday, it was revealed there had been a rise in the numbers of people admitted to intensive care with flu.
Government figures show that 302 beds are now occupied by flu patients.
Professor Peter Openshaw of the National Heart & Lung Institute, Imperial College London, said there was no evidence the virus had changed.
"From around the country, reports from frontline staff are showing unprecedented levels of hospitalisation with severe flu in high-risk adults.
"All the evidence we have so far is that the virus has not changed.
"It is affecting the same type of person as last year and still behaves like swine flu rather than normal seasonal flu (which mostly affects the over 65s)."
A Department of Health spokesperson said the figures were "in keeping with what we would expect during a winter flu season".
"But everyone can do their bit to help keep well - simple measures like washing your hand help stop flu spreading.
"The Chief Medical Officer has issued clear advice to get the seasonal flu jab If you are in a vulnerable group, particularly pregnant women and people with underlying health condition, as well as those aged 65 and over."
The latest information on the number of deaths from seasonal flu and swine flu will be released by the Health Protection Agency on Thursday.
Updated figures on levels of seasonal flu circulating in Scotland are also due to be released that day.