Home-grown winter flu is dying out. The seasonal infection that has caused thousands to take to their beds and wreaked annual havoc in the NHS for generations is losing its grip.

Latest figures show that the level of flu this winter is running at a historic low of 10 to 15 cases per 100,000 population, a rate more usually seen in the summer months. For the past five years the usual winter surge in flu cases has failed to materialise.

As the threat from avian flu moves closer, and European governments take steps to defend themselves against an expected pandemic, home-grown flu is in retreat. Experts say the two developments are linked. The drop in winter flu cases to a record low over the past five years is the culmination of a steady decline in the impact of the illness stretching back three decades to the last flu pandemic in 1968.

The reason is that the virus that first emerged in 1968 - H3N2 - has been around for so long that most people now have some immunity to it. Its capacity to lay people out, hugging their hot water bottles and cups of hot lemon, has been diminished.

Each year the virus undergoes a small mutation - called genetic drift - which creates a new strain capable of infecting a new generation of victims. But with each winter's illness, sufferers build up immunity that gives them some protection against next year's strain.

Sir John Skehel, director of the National Institute for Medical Research and an expert on flu, said: "More and more of us have been infected with [flu] viruses like the initial [1968] virus several times. Immunity has been built up and the virus doesn't have as many possibilities of changing.

"Most infections are in the new born who don't have immunity and older people whose immune systems are failing."

A surge in cases is expected after Christmas because as family and friends come together over the holiday, infection rates increase. But this year, fears over avian influenza triggered a run on flu vaccination which is expected to rise to record levels among the elderly, curbing the number of new cases of winter flu.

Once in a generation, the virus undergoes a major mutation - genetic shift - which creates a wholly new virus to which no one has immunity. That triggers a pandemic whose aftershocks are measured in successive winter epidemics that may last decades.

The current outbreak of avian flu - H5N1 - which claimed the lives last week of three children in the same family in eastern Turkey - may be the trigger that starts the next pandemic, experts have warned.

The infection is widespread in poultry flocks in the Far East where families live closely with their fowl, providing the opportunity for the human and avian viruses to mix and create a new pandemic strain. There have been 140 confirmed cases of avian flu and 60 deaths but no proven cases of human-to-human transmission.

Although winter flu is currently at its lowest level it was impossible to say whether this would continue, Sir John said.

"I would like to think that but we don't know. We have had too little experience of the virus since it was isolated in the 1930s. There have been only two pandemics since then - in 1957 and 1968 - and the 1968 one is still going on.

"I would like to think that cases of flu will continue to diminish. But it may be the virus comes under so much pressure that it does something unexpected. We don't know."

A second virus, H1N1 - the cause of the 1918 pandemic which killed an estimated 40 million people worldwide - returned in 1977 and caused new outbreaks of illness. The origin of that virus remains a mystery.

Sir John said: "It was probably an experiment going on somewhere in the east and some virus was accidentally released."