The newest avian influenza virus, (A) H5N1, is of particular concern for several reasons:

Direct transmission: H5N1 became the first known bird flu strain to jump directly from birds to people when it surfaced in Hong Kong in 1997. It has since infected people in other Southeast Asian countries. Two other strains have caused illness in humans, but neither is as severe as H5N1.
Virulence: The virus is especially lethal, killing close to 100 percent of susceptible birds and more than half of infected people. Birds who do survive can shed the virus for at least 10 days, greatly increasing the flu's spread.
Rapid spread: Since 2003, hundreds of millions of birds have died, a loss that's ecologically and economically devastating. It's also alarming from a public health standpoint — widespread infections among birds may lead to more human disease.
Genetic scrambling: (A) H5N1 mutates quickly and is notorious for grabbing large blocks of genetic code from viruses that infect other species, a process called reassortment. For that reason, it has particular potential to combine with a human flu virus, creating a new viral strain that spreads rapidly from person to person. The emergence of such a virus would mark the beginning of a potentially devastating pandemic. (Source)

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