WHAT IS A QUICK GUIDE? Quick guides are concise explanations of topics or issues in the news. More Quick Guides

Bird-flu viruses occur naturally in migratory wildfowl, though they tend not to develop an infection.

However the virus can be passed to domestic birds, which are far more susceptible.

The virus currently causing concern is the deadly strain H5N1, which can prove fatal to humans.

How does it affect humans?

Although the H5N1 strain of bird flu does not cross species easily, it can pass to humans.

About half those who have been infected by H5N1 - usually only after very close contact with birds - have died.

Symptoms range from fever, sore throat and cough to respiratory illness and organ failure.

Is there a global threat?

Lethal global flu epidemics tend to occur three or four times a century.

Some scientists believe a new one may be imminent and could be triggered by bird flu.

The concern is if people already suffering from a human strain of flu contract the H5N1 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.

Is there a vaccine?

WANT TO KNOW MORE? Have we missed anything out? Do you want to suggest a subject for a Quick Guide? Send us an e-mail

There is currently no effective vaccine to protect humans against H5N1.

If a pandemic virus emerges, it would take at least three months to start work on a vaccine.

However, some anti-viral drugs can help limit symptoms and reduce the chances of the disease spreading.