Avian influenza (or bird flu) is a highly infectious disease. It affects many species of birds, including chickens, ducks, turkeys and geese.
The disease can be passed between commercial, wild and pet birds, and can kill them.
Bird flu is caused by a flu virus that is closely related to human flu viruses.
There are 16 types of bird flu, but the type that has caused concern in recent years is the deadly H5N1 strain.
The H5N1 virus does not usually infect people, but infections with this virus have occurred in humans around the world.
Human cases
Since 2003, the World Health Organization (WHO) has confirmed over 400 cases of H5N1 in humans in Azerbaijan, Bangladesh, Cambodia, China, Djibouti, Egypt, Indonesia, Iraq, Lao People's Democratic Republic, Myanmar, Nigeria, Pakistan, Thailand, Turkey and Vietnam. This has led to over 250 deaths.
All these people are thought to have contracted the virus following close or direct contact with infected birds.
Millions of poultry have been killed in South East Asia and other countries during outbreaks, to prevent the disease from spreading among birds and to stop it being passed on to people.
Risks
The concern is that the H5N1 strain may undergo genetic changes that could enable it to spread easily from person to person. If these changes occur, there would be a greater risk to people.
At the moment, the H5N1 virus does not infect humans easily, and if a person is infected, it is very difficult for the virus to spread to another person.
Current situation
Since the H5N1 strain of bird flu emerged, it has been confirmed in birds in many countries, including the UK.
Although it is still present in other countries, the UK became officially free from bird flu on November 20 2008.
There are currently no confirmed reports of H5N1 being easily transmitted from person to person.
