Of the flu viruses that have recently emerged, a subtype called H5N1 is the cause of greatest concern, after demonstrating on two occasions a capacity to directly infect humans, most recently in Vietnam in January this year. The virus, thought to be transmitted through contact with bird droppings, spread at incredible speed through poultry populations, and its mutation to a form that can be carried by pigs and cats suggest that conditions are ripe for a devastating pandemic.

If more humans become infected over time, the likelihood increases that humans, if concurrently infected with human and bird flu strains, could serve as the “mixing vessel” for the emergence of a novel subtype with sufficient human genes to be easily transmitted by air. This would mark the start of an influenza pandemic.

The speed of H5N1’s mutations have stopped experts from producing an effective vaccine to date. Scientists are working on a human vaccine using the flu strain that killed people in Vietnam last winter. It is scheduled to go into trials early next year, but if the circulating H5N1 strain changes significantly the vaccine could be rendered useless.

Flu pandemics historically occur every 20 to 30 years, when the genetic makeup of a strain changes so dramatically that people have little or no immunity built up from previous flu bouts.

In the 20th century the great “Spanish flu” pandemic of 1918–19, which caused an estimated 40 to 50 million deaths worldwide, was followed by incidents in 1957–58 and 1968–69.

All the worst outbreaks originated from birds. Migratory birds, which have begun their annual escape from the northern winter for warmer Asian climes, carry the virus without necessarily dying of it.

That, experts fear, means that it might never be eradicated.