This is an overview of how the virus has spread from its initial source in Asia, from 1997 onwards.
1997: The first occurrence of the H5N1 virus on humans led to 18 people being hospitalised and 6 of them felling victim to it. 1.5 million chickens were killed in an attempt to eliminate the source of the infection. One conclusion however was that the virus spread mainly from birds to humans, while person-to-person infections were rare.
1999: 2 children were infected with the H9N2 form of the virus. They both made a full recovery and no further infections were reported. Again, all clues pointed to the poultry being the source of infection and it was direct contact with birds that transmitted the virus to humans. A few other H9N2 infections were discovered in Mainland China during the same period
2003: In Hong Kong, two people returning from China were infected with the H5N1 virus, one of whom died and the other making a full recovery. The source or infection was not determined, however another member of the same family, living in China, died following a respiratory illness.
2003: Netherlands had to deal with an outbreak of avian influenza A (H7N7), which affected poultry workers and their families, following a spread of the virus among chickens. Although the human cases of infection were mild and mostly manifested by eye infections, 80 cases were reported and 1 of the victims eventually died. Direct human-to-human transmission of the virus was proven.
2003: A child in Hong Kong was infected with the H9N2 strain of the virus; he received medical care and eventually recovered.
2003: Vietnamese authorities suspected and later confirmed the presence of the highly pathogenic H5N1 form of the bird flu virus in poultry. Its transmission to humans was also confirmed.
As well as further outbreaks of the virus throughout Asia (cases were reported in Japan, The Republic of Korea, Thailand, China, Cambodia, Pakistan and Indonesia), laboratory studies determined that the H5N1 virus has suffered mutations from its form in 1997 and also that it was immune to certain antiviral drugs, like M2 inhibitors Amantadine and Rimantadine.
2004: H5N2 virus infected poultry were reported in Texas, USA. However quick action by the local and federal authorities prevented its spread and there were no other animal or human cases found.
2004: Felines were affected by the avian influenza, as domestic cats in a Thailand household fell victim to it and tigers kept in a Thai zoo also contracted the virus later on. Vietnam reported further human cases of infection with the virus.
2005: As more wild birds were found dead and tested positive for avian influenza on some important Asian migratory paths, the virus broke out in European countries. H5N1 infested poultry and wild birds were found in Turkey, Romania, Greece, Ukraine, Russia and Croatia. The World Health Organization's report announced 122 human cases had been reported so far, with 62 people dying as a direct consequence.
2005: Plans to produce a bird flu vaccine were announced, while in Vietnam abnormal medical results in 14 avian influenza human victims indicated it might be possible to predetermine if the disease will be fatal.
2006: The first known human occurrences of the H5N1 virus were found in Turkey and by the 9th of January 14 people had been diagnosed with bird flu. Some 700,000 fowl were killed to contain the virus, whilst no indication of a spread between humans was found.