Young adults are being struck down with the flu at three times the rate of their parents' generation, figures from heath watchdogs reveal.
For every 100,000 people aged 15 to 44 in England, 54.4 of them have the flu, compared with 18 cases among the same number of people aged over 65.
Doctors have been diagnosing more cases of flu during this year's lead-up to Christmas than they have done during the past 10 pre-Christmas periods. The overall rate stands at 39 cases of influenza per 100,000 people within the general population, the government's Health Protection Agency said.
The number of flu cases reported to GPs has already breached the minimum "baseline" figure that marks the point at which the outbreak is defined as seasonal but it is still well short of the 200 cases per 100,000 that turn an outbreak into an epidemic.
Health experts said the rise in cases among the young, particularly in southern England, was in part due to the fact that many had not received the flu vaccine to protect them against the different strains of the illness.
And they said that flu outbreaks often affected certain demographics differently. They suggested that young working people suffering from the early symptoms of flu stay at home, rather than risk spreading the virus through their workplace.
Even with flu-sufferers staying at home, the working population is still likely to experience an increase in the number of cases of flu.
The current outbreak is likely to reach its peak across all age groups within the next three to four weeks, said Douglas Fleming, the director of the Birmingham Research Unit of the Royal College of General Practitioners.
"We expect to see an increase in the number of cases within that age group, but also possibly in other age groups as well," Dr Fleming said.
"There is often a tendency for the virus to affect one age group more than another.
"The reason has a lot to do with the amount of mixing within any given age group, but it is also connected with the properties of the virus.
"We expect to see a figure that is much higher within this young-adult age group when the latest figures come in this week."
The flu epidemic in the winter of 1999-2000 mainly affected older people while the epidemic in 1989 mostly affected younger age groups, Dr Fleming said. "We are now looking at a maximum impact on the working age groups."
Richard Pebody, the head of infectious disease surveillance at the Health Protection Agency, said: "There is higher activity in the working-age group. Often we see one age group or other being more affected, so it is not unusual.
"The simple message is, if you do get flu it's best to stay at home to avoid spreading the virus in the office."
Scientists said that the H3N2 strain of the influenza virus was causing the current flu outbreak.
The outbreak began in central England and has since spread to the rest of the country.
The flu vaccine, which is offered free to everyone over the age of 65, covers the H3N2 strain, in addition to two other flu viruses.
The low incidence of flu in people over 65 indicates that the vaccine is limiting the spread of the virus. However, 27 per cent of over 65s have not been vaccinated against the illness.
In the under 65s, certain high-risk groups such as people with asthma or heart disease are also offered the flu vaccine for free, although only about four in 10 people within these high-risk groups have taken up the offer.