Also see the WHO timeline of major H5N1 events for details regarding specific dates in avian flu development.

First human A/H5N1 infection
Influenza A/H5N1, a severe strain of bird flu, was first found to infect humans during poultry outbreaks in Hong Kong in 1997. Eighteen humans were infected and six died. A mass culling of all poultry in Hong Kong may have prevented a human A/H5N1 pandemic (global epidemic) at that time.

Wave I: December 2003 to April 2004
In December 2003, A/H5N1 re-emerged, causing outbreaks in commercial poultry farms in South Korea. By early January 2004, Vietnam reported unusually severe respiratory illnesses in 11 children in Hanoi, which was later confirmed as due to H5N1. Days later highly pathogenic bird flu H5N1 was confirmed in poultry in the country, and by the end of the month, there were more than 400 outbreaks. In the same month, Japan detected the disease in a poultry farm in Kyoto. Thailand then confirmed two human cases at the same time as a poultry farm infection. By the end of January, Cambodia, Laos, Indonesia and China reported outbreaks in poultry.

No avian influenza had ever infected as many countries simultaneously. During this period, a total of 35 human cases occurred, in Thailand and Vietnam, of which 24 were fatal.

Wave II: June to October 2004
China, Indonesia, Thailand and Vietnam reported return of the disease in poultry. Malaysia detected outbreaks in August. Thailand and Vietnam again reported human cases during this period - a total of 9 cases, only one of whom recovered.

Despite control efforts, the virus had become endemic (permanently present) in parts of Asia.

Wave III: December 2004 to the present
Ongoing outbreaks in poultry continued in Indonesia, Thailand and Vietnam through 2005. Human cases occurred in Thailand, Vietnam, and for the first time Cambodia.

In April 2005, in the Qinghai Lake district of China, over 6,000 wild birds of many different species suddenly died from H5N1. This lake is a major stop for many migratory birds, and it is thought that this event contributed significantly to the apparent sudden subsequent spread of H5N1.

Migratory birds, and possibly trade in poultry, have since spread the virus to parts of Russia, the Indian subcontinent, the Middle East, Europe and Africa.

Virus has changed genetically
The H5N1 virus is now genetically different than the one first detected in Hong Kong in 1997. The current strain is known as the Z strain. Studies of the virus show that H5N1 is progressively more pathogenic, can survive longer in the environment and is able to infect an expanding range of animals (pigs, tigers, cats, dogs).

Infection in humans
The picture of H5N1 infection in humans is still evolving. It can cause severe illness in humans, with an overall mortality rate of greater than 60%. The vast majority of cases have occurred among people who work on or live near poultry farms. More children than adults have been infected. Clusters of cases in family members have been noted, and there have been documented cases of human-to-human transmission, for up to two "generations" of spread. See the details about the world's largest cluster to date, which happened in May 2006, on the Indonesia page of this website.

Summary of countries affected and humans cases
As of October 2009, 62 countries have reported birds (wild and domestic) infected with A/H5N1. Human cases have occurred in 15 countries: Azerbaijan, Bangladesh, Cambodia, China, Djibouti, Indonesia, Egypt, Iraq, Laos, Myanmar, Nigeria, Pakistan, Thailand, Turkey and Vietnam. There have been 262 deaths from 442 confirmed cases since December 2003. See Maps and Country Details on this site for full details. Indonesia has not confirmed any cases thus far in 2009, though it is likely human cases have continued to occur in that country. Reporting is unreliable.

Will A/H5N1 Cause the Next Human Pandemic?
Several influenza pandemics occur each century and another will certainly occur at some time in the future. The erruption of the pandemic H1N1 ("swine flu") virus in May 2009 in no way diminishes the risk that avian flu A/H5N1 could cause a pandemic.

For A/H5N1 to cause a pandemic, it must gain the ability to spread easily from human to human; this is possible as influenza viruses are renowned for their ability to spontaneously change their genetic structure. If A/H5N1 gains this ability, it could cause a pandemic similar in severity to the 1918-19 "Spanish Flu".

Lethal to domestic poultry, no symptoms in wild birds
Influenza A/H5N1 is "highly pathogenic", causing a severe disease with virtually 100% mortality in domestic poultry. The disease can be carried without symptoms in wild birds. However, once it infects susceptible birds, the outbreak generally is very dramatic.

Transmitted in respiratory secretions and feces
The virus is highly contagious in poultry, transmitted bird-to-bird through direct contact with infected respiratory secretions and feces.

Symptoms in birds
The birds suddenly become unwell, and may die quickly without showing any particular symptoms. Symptoms which may occur include ruffled feathers, loss of appetite and decline in egg production. They become weak, stagger, have diarrhea, develop swollen faces, their combs turn blue, small bleeding points on the skin may be noted, and they have laboured breathing. Death can occur within 1-2 days, and almost the entire poultry flock is affected.

Outbreaks unreported
The Food and Agriculture Organization (FAO) stresses that absolute transparency about disease outbreaks is vital to controlling the disease among birds and, consequently, managing the risk of a human pandemic. However, many outbreaks of avian flu or suspected flu remain unreported as farmers depend on their flocks for food and money. Unless they are compensated for birds they cull, some farmers they are in the unenviable position of deciding whether to kill birds as a precautionary measure and lose their livelihood, or to continue rearing sick birds and potentially allow avian flu to flourish among their flocks and maybe even their family members. Farmers also may reject poultry vaccination, as some countries limit import of vaccinated poultry. Instead of trying to ban people from raising birds, the FAO suggests encouraging farmers to participate in movement restriction or vaccination campaigns.

More information
For further information see the World Organization for Animal Health Description of Avian Influenza.

First human A/H5N1 infection
The first recorded human infection with avian flu A/H5N1 occurred in Hong Kong in 1997. Eighteen people were infected and six died.

Occurred in waves
There were two waves of infection - one case in May, the other 17 cases occurring from November to December. Outbreaks in poultry farms and live markets were detected at the same time, and most human cases were found to have had direct exposure to live poultry.

Possible limited human-to-human spread
Two healthcare workers who looked after patients with H5N1 were later found to have "seroconverted" (developed antibodies), indicating limited human-to-human transmission may have occurred.

Did culling prevent a global pandemic?
Public health authorities ordered all poultry in Hong Kong to be culled (killed), and no further human cases occurred. Some experts feel that this culling may have prevented a A/H5N1 pandemic.