Avian or bird flu is caused by a new type of influenza virus (H5N1) that has emerged in south-east Asia in recent years. It is endemic in chickens and ducks in countries such as Vietnam and Thailand.
Initially it was believed the virus could only be transmitted by direct contact between an infected bird and a person. However, there is now evidence of direct human-to-human transmission.
What makes the H5N1 strain of flu so dangerous is its ability to mutate rapidly and to pick up genes from other species. Experts believe it is only a matter of time before it increases its infectivity among humans.
With the advent of rapid, long-distance air travel, it is likely that an outbreak of highly infectious avian influenza will spread rapidly to Europe, the US and Australia. The pandemic could also emerge during the summer or winter, outside the traditional flu season.
Because the H5N1 strain is so different from the usual winter influenza outbreaks, previous vaccinations against flu will not be sufficient to protect someone from avian flu. And it could take up to six months to manufacture a vaccine specifically against a new strain.