THERE IS not much to look forward to in January and February other than snowdrops and, for some people, a Valentine card. Flu is almost as much part of the scene as the snowdrops. Both last year’s flu season and, so far, this year’s have been unexceptional in the UK.

But in Thailand and Vietnam the story has been different. Emerging in these countries are the first signs of an outbreak of avian influenza that could rock the world. The strain of avian flu that is causing the worry is influenza A (H5N1). This strain is notoriously liable to be lethal in human beings and has the potential for starting a worldwide outbreak, a pandemic.

There was an outbreak in Hong Kong in 1997 and a smaller, Far Eastern outbreak in 2003.

There is evidence that since the 1997 outbreak there has been a drift in the nature of the influenza A (H5N1) that has made it more virulent, capable of causing a higher death rate in the human population and of infecting a greater range of other mammalian and bird hosts.

Both The New England Journal of Medicine (NEJM) and the BMJ have recently had disturbing editorials on the danger that this strain of avian flu could pose if it became established in people and human-to-human transmission resulted. Whenever a totally new strain of flu emerges, a pandemic is likely to follow: because the population has no resistance to a recently emergent virus, the infection usually sweeps through a community.

The most notorious pandemic to date was the flu epidemic during and after the First World War when so-called Spanish flu killed 25 million in six months.

Other serious pandemics of flu followed the first recorded probable one in 1493; this may have been associated with infection in pigs carried across the oceans by Christopher Columbus. Pigs were also implicated in the 1918 pandemic. Other recent serious pandemics occurred in 1957 and 1968.

When influenza A (H5N1) has crossed the species barrier and has been caught by human beings, it has caused a higher death rate among the sufferers than that which followed infection by Spanish flu. Of the 44 people who caught avian flu in the recent Far Eastern outbreak, 32 died.

As well as a high mortality rate, the current strain of avian flu tends to infect young people and children who had previously been in good health.

Avian flu in people causes a viral pneumonia that is potentially fatal, even before there is evidence of secondary bacterial infection.

The NEJM, while expressing obvious deep concern about the situation, finds a ray of hope. In the densely populated parts of Asia more than 120 million poultry birds either died or were destroyed between January and March last year as part of a great effort to stem the tide of avian flu. People working on this programme were exposed to the virus but, perhaps surprisingly, there was neither a general outbreak among the workers nor evidence of human-to-human transmission. The hope is that the explanation for this, whatever it might be, may protect the world from a catastrophic pandemic.

Avian influenza A (H5N1) has been isolated in diseased pigs. It has been found most often in domestic poultry, both chickens and ducks, which in the Far East live in close proximity to those looking after them. This custom is a potential encouragement to the emergence of strains of flu capable of human-to-human transmission.

Wild ducks are a natural reservoir of the influenza viruses but, although they are carriers of the virus, do not usually show evidence of suffering from the disease. Surprisingly, the lethal form of avian flu has also been detected in dead migratory aquatic birds. The discovery that domestic ducks can excrete large quantities of the lethal form of avian flu without suffering from disease adds another factor that complicates the understanding of the natural history of this virus.

A greater understanding of the virus is an essential prelude to developing tactics that may restrict the spread of the disease. The NEJM suggests that we need answers to several questions about the behaviour of the virus and its epidemiology before public health authorities can introduce rational disease-control measures but that, in the long term, we need a vaccine that is potent against a wide spectrum of flu viruses.