How Bird Flu Kills

In 1997, the H5N1 virus killed six of 18 people infected with it.

It took five years to determine how the 1997 strain of H5N1 kills. It's deadly because it provokes an extreme response from the human immune system. It contains a mutant gene that makes it resistant to the immune response.

When someone becomes infected, the body pumps out as much as seven times the amount of cytokines as it does with less severe flu strains. These proteins invoke the white blood cells, called macrophages, to destroy the virus. In an extreme response, the macrophages destroy red blood cells instead. Patients at first appear to have normal flu symptoms, but can within hours decline and go into a coma.

Human-to-human transmission of the virus has never happened, but experts are taking no chances. The headquarters of the World Health Organisation in Geneva initiated its Global Influenza Surveillance network. They're worried about the potential of this new virus mutating into pathogenic form. If that happened, it would be able to jump person-to-person.

Not good in a crowded, international city.

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