H5N1 Virus:

Influenza A virus subtype H5N1, also known as A(H5N1) or H5N1, is a subtype of the Influenza A virus that is capable of causing illness in many animal species, including humans. A bird-adapted strain of H5N1, called HPAI A(H5N1) for "highly pathogenic avian influenza virus of type A of subtype H5N1", is the causative agent of H5N1 flu, commonly known as "avian influenza" or simply "bird flu", and is endemic in many bird populations, especially in Southeast Asia. One strain of HPAI A(H5N1) of Asian lineage is spreading globally. It is epizootic (an epidemic in nonhumans) and panzootic (a disease affecting animals of many species, especially over a wide area), killing tens of millions of birds and spurring the culling of hundreds of millions of other birds in an attempt to control its spread. Most references in the media to "bird flu" and to H5N1 are about this specific strain.

HPAI A(H5N1) is an avian disease, and there is no evidence of efficient human-to-human transmission or of airborne transmission of HPAI A(H5N1) to humans. In almost all cases, those infected with H5N1 have had extensive physical contact with infected birds. However, around 50% of humans known to have been infected with the current Asian strain of HPAI A(H5N1) have died from H5N1 flu, and H5N1 has the potential to mutate or reassort into a strain capable of efficient human-to-human transmission. In 2003, world-renowned virologist Robert Webster published an article titled "The world is teetering on the edge of a pandemic that could kill a large fraction of the human population" in American Scientist calling for adequate resources to fight what he sees as a major world threat to possibly billions of lives. On September 29, 2005, David Nabarro, the newly-appointed Senior United Nations System Coordinator for Avian and Human Influenza, warned the world that an outbreak of avian influenza could kill anywhere between 5 million and 150 million people. Experts have identified key events (creating new clades, infecting new species, spreading to new areas) marking the progression of an avian flu virus towards becoming pandemic, and many of those key events have occurred more rapidly than expected.

Due to the high lethality and virulence of HPAI A(H5N1), its endemic presence, its increasingly large host reservoir, and its significant ongoing mutations, the H5N1 virus is the world's largest current pandemic threat, and billions of dollars are being spent researching H5N1 and preparing for a potential influenza pandemic. At least 12 companies and 17 governments are developing pre-pandemic influenza vaccines in 28 different clinical trials that, if successful, could turn a deadly pandemic infection into a nondeadly one. Full-scale production of a vaccine that could prevent any illness at all from the strain would require at least three months after the virus's emergence to begin, but it is hoped that vaccine production could increase until one billion doses were produced by one year after the initial identification of the virus.

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