At A Glance
Avian flu has devastated bird populations around the globe. But if the virus acquires the ability to jump easily between humans it could unleash a pandemic, killing millions of people within months.
- No cure exists
- Developing countries most at risk
Hundreds of people have died around the world from bird flu. Almost all have caught it from birds, but scientists believe there have been a few cases of human-to-human transmission.
A human pandemic would be particularly catastrophic in developing countries where living conditions and malnutrition are likely to make people more vulnerable, health services are weak, and vaccines and antiviral drugs would be beyond reach.
Even if there is no pandemic, bird flu threatens the livelihoods of millions of people in Asia and Africa. Hundreds of millions of domestic birds - most owned by poor farmers in developing countries - have died or been culled, and some countries have banned imports.
Only a few kinds of bird flu infect humans, and only a few hundred people have died from the disease. But scientists fear that if the virus mutates into a human disease, it could kill millions within a few months.
The largest and most severe outbreak on record originated in southeast Asia in 2003, and has since spread across the Middle East, Europe, Africa and other parts of Asia.
It is caused by the H5N1 virus which has threatened the livelihoods of millions of people. Health officials have destroyed flocks and other countries banned imports. Disease and culls have killed hundreds of millions of birds in dozens of countries.
The virus was dubbed "chicken Ebola" because it causes massive internal bleeding, and can wipe out a flock within hours.
Although the number of human cases has fallen since 2003, scientists are still concerned about the H5N1 strain because it circulates widely in some poultry populations and most humans have no immunity to it.
Other types of bird flu which have infected people include avian H7 and H9.
The most recent strain to infect humans is the H7N9 virus, that emerged in China in March 2013.
Risk to humans
For the moment, the risk of catching bird flu is minimal. Despite the infection of tens of millions of birds over large geographical areas, the number of humans who have contracted the disease is in the hundreds. See the World Health Organization for updates on the outbreaks.
Close contact with sick or dead birds is the main source of infection.
The risk of catching the virus appears greatest during the slaughtering and preparation of infected birds for eating. In a few cases, children are thought to have caught the disease through exposure to chicken faeces while playing in an area used by poultry.
Symptoms in humans include fever, coughing and breathing problems. Pneumonia and multiple organ failure are common. Other symptoms can include diarrhoea, vomiting, abdominal or chest pain, and bleeding from the nose and gums.
The World Health Organization (WHO) says there have been a tiny handful of cases where one person has infected another, but at the moment the virus cannot pass efficiently between people.
However, if it acquires the ability to spread the way normal flu does, it could be devastating, both because it is so pathogenic and because humans have no immunity to it.
No one can say when this might happen - it could be tomorrow, or in a decade, or indeed never. It is also possible the virus could lose its dangerous qualities as it mutates.
A human pandemic could start in two ways. If a person were infected with avian and human flu viruses at the same time, they could swap genes creating a new strain transmissible between humans. In this case it would spread very rapidly.
Alternatively, the virus could adapt more gradually, improving its ability to bind to human cells during repeated human infection.
WHO has estimated that even a mild form of the disease that can be passed between humans would kill 2 million to 7.4 million people.
A serious pandemic would send the world into recession as workers become ill and others stay at home to care for them or avoid infection. It would disrupt trade, transport and food supplies. Extra costs would include hospital and medical treatment and measures to contain the disease in birds.
Is there a cure?
Poultry can be vaccinated against the virus. But it is impossible to predict which strain will cause a human pandemic, and it would take months to make a new flu vaccine once the virus has appeared.
Human flu vaccines have been developed for H5N1 but are not yet ready for widespread use. Scientists are working on a vaccine for the latest H7N9 strain.
Many countries have stockpiled antiviral drugs which may improve survival prospects if taken within 48 hours of symptoms appearing.
Antivirals were developed to treat seasonal flu and work by preventing the virus from replicating itself. But no one really knows how effective they would be against a bird flu pandemic.
Some developing countries have produced cheap generic versions of the drugs, but the cost is still too high for many at risk of the virus.
The spread of the virus
Scientists believe a massive increase in Asia of human and bird populations living alongside one another helped the disease to cross the species boundary.
The first human cases of H5N1 occurred in Hong Kong in 1997 when 18 people were infected and six died. Hong Kong's entire poultry population was slaughtered.
H5N1 reappeared in poultry in southeast Asia in 2003, and later spread to Central Asia, Africa, the Middle East and Europe.
There are hundreds of strains of bird flu. Many wild birds carry flu viruses with no apparent signs of harm, but other bird species including poultry develop disease when infected.
Migrating wildfowl are believed to be responsible for spreading the virus westwards, but trade in live poultry may have played a role in its movement around Asia.
Donors have given millions of dollars to combat the spread of bird flu.
The arrival of bird flu in Nigeria in early 2006 raised serious alarm. Since then it has spread to other parts of the continent.
Millions of households live alongside chickens throughout the region, increasing the chances of the virus crossing into humans.
Poor medical, veterinary and laboratory services, lack of health education, porous borders and high mortality rates from other diseases mean a new human virus could easily spread undetected.
Farmers and villagers, many unaware of the risks, dispose of infected birds without protective masks and clothing.
But even with a concerted education campaign, many are likely to continue selling or eating birds that have died because they cannot afford to throw away meat even if it might be infected.
The World Health Organization (WHO) provides all the latest updates on bird flu. This fact sheet is a good place to start for an introduction to the virus in birds and humans.
If you want to know more about the impact of the disease on those whose livelihoods depend on poultry, try the U.N. Food and Agriculture Organisation's bird flu site.
The World Bank's avian flu website provides information on programmes to prevent the disease and describes the potential economic impact.
U.N. news service IRIN has a flu section that includes reports on bird flu from around the globe.
The European Centre for Disease Prevention and Control monitors outbreaks of bird flu.
The World Organisation for Animal Health, set up in 1924 to help countries work together to eradicate diseases threatening their livestock, keeps its site updated with news of the latest outbreaks. It also has information in French and Spanish.
(The following looks at the initial spread of the virus and focuses mainly on developments affecting Asia and Africa. It doesn't include every human death or outbreak in poultry. Bird flu refers to the H5N1 strain.)
1996 - H5N1 detected in a goose in Guangdong province, China
1997 - First documented outbreak of human H5N1 infection. Six of the 18 people infected die. Hong Kong's entire poultry population is destroyed
Dec - South Korea confirms bird flu at a chicken farm near Seoul and begins mass cull
Jan - Vietnam says bird flu found in poultry and confirms human cases. Thailand confirms its first human cases
Sep - First case of one human infecting another occurs in Thailand
Feb - Cambodia reports its first human case
Apr - Migratory birds, infected with H5N1, begin dying in Qinhai Lake, central China. Viruses detected later in 2005 in countries along migratory routes are found to be almost identical to the Qinhai Lake virus
Jul - Indonesia reports its first human case
Jul/Aug - Russia and Kazakhstan report outbreaks in poultry. Dead migratory birds reported in vicinity
Oct - First H5N1 cases in poultry in Turkey, prompting EU to ban imports of live birds and feathers from Turkey. H5N1 confirmed in ducks in Romania - first case in mainland Europe
Nov - Kuwait reports H5N1 in a flamingo - first known case in Gulf Arab region. China reports its first two human cases (but see below)
Feb - First African cases of H5N1 detected in poultry in northern Nigeria. India and Egypt also find first cases. France confirms H5N1 at a farm where thousands of turkeys have died - the first cases in farm birds in the EU. Tests show H5N1 in domestic ducks in Niger
Apr - Ivory Coast detects its first outbreaks in birds - the sixth African nation to confirm the virus after Nigeria, Niger, Egypt, Burkina Faso and Cameroon
Aug - China says its first H5N1 human case was in 2003, not 2005 as it originally reported
Feb - WHO confirms bird flu has killed a Nigerian woman - the first known human fatality in sub-Saharan Africa