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The H5N1 influenza virus, which was first identified in South African terns in 1961, is classified as a "highly pathologic avian influenza," or HPAI -- a variant of the flu defined by its ability to cause severe outbreaks of lethal disease in bird flocks.
The presence of the H5 or H7 surface protein generally indicates an HPAI, and H5N1 is no exception. Since the current strain emerged in Hong Kong in 1997, it has mutated a number of times, with a variant known as Z+ proving to be incredibly virulent, killing almost 100 percent of infected chickens. It has also shown a disturbing proficiency at infecting a wide range of mammals, including cats, which are generally considered resistant to influenza.
H5N1 also has the unusual ability to attack humans directly, often with fatal results. Since 1997, it is suspected to have infected hundreds of people, and killed at least 62 as of early September 2005. In humans, it can be incredibly destructive. While most common flu deaths are caused by opportunistic infections (like bacterial pneumonia, which takes advantage of an immune system weakened by the flu), the current strain of H5N1-- like the 1918 pandemic strain -- is powerful enough to kill directly, either through infection of the nervous system or by viral pneumonia (as seen in the X-rays above -- the white clouding indicates progressive cellular damage to the lungs of a flu patient), which results in an overreaction from the immune system that destroys the lungs.
Given this virus subtype's potency, it is feared that if a recombination or reassortment gives it the ability to move easily between human beings, it could trigger a massive pandemic -- and for some scientists, the question is not if a pandemic will erupt, but when. If the virus maintains its virulency, the death toll could rival or exceed that of the 1918 "Spanish" flu, potentially causing tens of millions of deaths.
Credit: Centers for Disease Control
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Briefing
Read a briefing on the dangers posed by avian influeza and the challenges nations must overcome in order to fight against it.
Interactive Map
Explore how avian flu is spreading across Asia.
Filmmaker Notes
Go behind the scenes with producer Micah Fink.
Info-Graphic
Learn how chemical and biological weapons work.
Interactive Challenge
Test your knowledge of the HIV/AIDS pandemic.
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