Avian influenza virus, strain H5N1, is the latest flu virus with the potential to trigger a pandemic outbreak of flu due to its high lethality in birds and humans. The human population has no known natural immunity to H5N1 because it is genetically distinct from the three flu strains currently circulating in humans (H1N1, H1N2, and H3N2). Bird disease and death from H5N1 have occurred throughout Asia where close interaction between people and poultry have resulted in 114 human cases and 59 deaths from late Dec. 2003 to Sept. 19, 2005 according to WHO. Hundreds of millions of farm birds have been killed throughout Asia in an attempt to control the spread of the virus. New influenza strains like H5N1 arise when two different viruses infect the same animal such as a bird or pig. Since influenza has a segmented RNA genome, individual segments can be exchanged between viruses in a single infected cell creating a new virus. This rare event is called a genetic shift.