![[ image: width=150]](/olmedia/images/_38867_flu3.gif)
![image: [ Children in Hong Kong have been told not to play with chickens ]](/olmedia/images/_38867_kids.JPG)
Three pandemics of flu have swept the world this century, in 1918, 1957 and 1968, killing millions of people. All were caused in the same way, when the virus altered its structure, thus preventing the body's natural immune structure from recognising, and attacking it.
So far the newly identified virus, H5N1, only seems to have spread from birds to people, but if begins to move from person to person past experience suggests it could start a worldwide epidemic.
This is because the virus contains new antigens, chemicals on its surface, to which humans have never been exposed. So even people resistant to existing strains of human flu may have little or no resistance to the new strain, and no existing vaccine is likely to protect effectively against it.
How the mechanism of transmission works
The flu virus is constantly changing the structure of its coat, which is what normally enables it to slip past the body's defences. "What we see happening every year is that the viruses are slightly changing ... that's why we need an annual update of the vaccine." says Dr Albert Osterhaus of the National Influenza Centre of the Netherlands.
As well as this gradual, but continuous change, sometimes the flu virus undergoes sudden bigger changes known as shifts. These happen when human flu viruses exchange genes with flu viruses that infect birds or animals.
![[ image: width=150]](/olmedia/images/_38867_pigs.jpg)
The swopping takes place in pigs, which can be infected by both human and bird flu viruses. South Asia has the largest concentration of pigs and poultry, and as such identified as a likely epicentre for the next pandemic.
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The foreign genes make foreign proteins, which appear on the surfaces of human virus particles and render them unrecognisable by the immune system. After a while it learns to recognise and attack the new strain, but in the meantime the new strain of virus has had time to spread.
"This particular virus actually carried one surface protein that had not been seen before and was apparently of avian origin," says Dr Albert Osterhaus.
The race to develop a vaccine
The World Health Organisation has been alerted and it has asked labs all over the world, which form an early warning network, to watch out for more cases.
Scientists in Britain, the United States and Japan have isolated the virus and are growing it in the laboratory.
If the virus does start to move from person to person, and larger-scale outbreaks start to occur, then this preparatory work will enable pharmaceutical companies around the world to start producing vaccine. It could cut the time needed to produce and distribute the new vaccine from around six months to around four months, and so save many lives.
WHO preparing vaccine against new 'bird flu' virus
(08 Dec 97 | World)
Guidelines for the prevention and treatment of flu
Influenza Bibliography
Centre for Disease Control
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