Lethal global flu epidemics tend to occur three or four times a century.
Scientists believe a new one may be imminent and is likely to be triggered by bird flu.
The latest H5N1 strain has killed millions of birds and scores of humans since 2003.
Many experts fear the virus will soon mutate into a strain more easily spread among humans.
A primary concern is if people already suffering from a human strain of flu contract the avian virus.
The viruses could join to create a previously unknown version not yet recognised by human immune systems.
The new variant is likely to spread quickly and easily, triggering a deadly pandemic.
Experts predict it could kill more than 2m people and infect millions more.
Bird-flu viruses occur naturally in migratory waterfowl, which tend not to develop symptoms.
But the virus can be passed to domestic birds, which are far more susceptible.
Once the virus is found in commercial or household flocks, rapid destruction is recommended of all birds that might be infected and farms quarantined.
Millions of birds have been killed in an effort to contain the latest outbreak and minimise risk to humans.
Although the present strain of bird flu does not cross species easily, it can pass to humans.
About half those who have been infected by H5N1 – usually after close contact with birds – have died.
But scientists say that any mutation may affect millions.
Symptoms range from fever, sore throat and cough to respiratory illness and organ failure.
There is currently no effective vaccine to protect humans against H5N1, although one is being developed.
Once a pandemic virus emerges, it would take at least three months to start work on a vaccine.
No-one knows what form the mutated virus would take.
However, some anti-viral drugs can help limit symptoms and reduce the chances of the disease spreading.