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 avian flu virus has a death rate of 70 per cent and while the eventual strain to emerge may have a lower mortality rate, past experience of flu pandemics points to significant levels of both disease and death.
The old and the chronically ill are likely to bear the brunt of the disease, although a second wave of the infamous 1918 Spanish flu killed millions of young adults.
Because the H5N1 strain is so different to 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.
We will therefore be dependent on the anti- viral drug, Oseltamivir, to prevent and treat avian flu. It is especially effective if distributed and taken prior to infection, with up to 90 per cent efficacy reported in clinical trials.