IN ALFRED HITCHCOCK’S CLASSIC movie The Birds, it is not so much the frenzied pecking of Tippi Hedren that chills the blood as the silent periods between the avian onslaughts. They are times of unnerving calm, of horrible uncertainty.

The storyline of the current outbreak of avian flu in the Far East, which started unfolding in December 2003, is equally disturbing, and the days of calm are not yet ending with good news. Of 31 people who are confirmed to have caught the current strain of bird flu (H5N1) in the Far East, according to the World Health Organisation (WHO), 22 have died. Although deaths have been reported only in Thailand and Vietnam, the flu outbreak is spreading through poultry in eight other countries, most of which have never reported this strain before. Tens of millions of birds have been culled in an effort to stem the tide of infection.

Now a United Nations organisation in Vietnam has reported that the virus has been found in pigs in Hanoi, although the news has been played down by the authorities and not yet confirmed by the WHO. The pigs, which showed no symptoms, apparently tested positive for H5N1 in nasal swabs; the WHO says that the presence of the virus in nasal cavities means the pigs may simply be contaminated but not infected (the virus has to defeat the animal’s immune system to take hold).

Although the virus does not yet appear to be able to jump from human to human — all those infected have caught it by handling poultry — its appearance in pigs is bad news. At best, it could mean that we are unlikely to see H5N1 consigned to the history books just yet. At worst, it is a nightmarish plot twist that has the potential to create a flu virus that can spread like wildfire in people. The most famous pandemic — the outbreak of Spanish flu in 1918 — killed at least 20 million people and more likely double that. At the time, the world population was only around 1.8 billion and mass tourism — which has spread diseases around the world at incredible rates — did not exist.

Pigs are a crucial part of the story because they can catch both bird flu and human influenza: it is possible that a pig could become a cocktail shaker for the avian and human strains. That creates a chance of the two strains genetically rearranging themselves into one highly pathogenic (disease-causing) hybrid virus. This daughter virus could be the worst of all worlds, combining extraordinary virulence with easy transmissibility. The current strain seems exceptionally nasty: it has killed 70 per cent of those infected, including young, fit individuals. Moreover, flu viruses are known to have poor photocopying machinery: each replication increases the odds that a nasty virus will emerge.

A hybrid virus would be transmitted not through the food chain — the virus is harboured in airways and lungs rather than muscle — but through the handling of live pigs. A pig with flu secretes the virus in the spray from its airways. A person in close proximity, for example during feeding, can then inhale the spray and become infected (just as human flu is passed on in the aerosols from sneezing). This scenario need happen only a few times to pose a risk: each infected person then starts a chain of infection among their friends and family. The transport of live infected animals would also spread the disease, both to other animals and the people handling them.

Karl Nicholson, Professor of Infectious Diseases at Leicester Royal Infirmary, says that farming practices in the affected countries raise concern that, even if the latest twist is unconfirmed, pigs could catch bird flu. He adds that, as far as the birds are concerned, it is also an outbreak of unprecedented virulence. “Viruses show a degree of fastidiousness, in that bird viruses tend to infect birds and human viruses tend to infect humans.”

Birds and people, he explains, have different receptors in their airways, and a virus primed to infect one species will generally not be able to infect the other. Pigs, however, are vulnerable to both.

Nicholson says: “The idea is that these viruses can mix together in pigs, because pigs are susceptible to both. Farming practices being what they are, you often have pigs and poultry close to each other. Sometimes they even live in the same household, with the farmer living upstairs and the animals living downstairs. There’s the fear that there will be some mixing in pigs.”

Previous research by Jeffery Taubenberger at the US Armed Forces Institute of Pathology has shown that the 1918 flu strain is unlike any other human strain but is very similar to a swine virus. That has lent support for the so-called “mixing vessel” theory — that to become really deadly in humans, avian flu must go via pigs, where it combines with portions of the human flu virus already present. However, there is no proof of this.

Virologists note a similarity between the 1918 virus and swine fever, but they cannot rule out the possibility that people infected pigs rather than the other way round.

Nicholson points out that pigs-as-mixing-vessels remains just a theory and that it’s just as plausible that people could be mixing vessels. After all, the current outbreak, and previous ones, show that people can catch bird flu directly from birds. That suggests that human beings would be capable of hosting the kind of genetic reassortment that could lead to a vicious virus.

The journal Science recently reported that biologists at the Scripps Research Institute in California confirmed that the 1918 strain was almost certainly avian in origin. The question is whether it jumped directly into human beings or went into pigs first. Either way, it is a salutary finding.