Flu primarily caused by viruses that normally infect only birds (e.g. H5N1 virus). Transmission is from bird to bird, but occasionally transmitted from bird to human. Fear of global pandemic would occur if the virus were to become capable of spreading from person to person as the human immune system would not recognize the virus and would require time to mount a defense. Unfortunately, the virus may be very virulent and kill the individual before the defense is capable of mounting. Death rates could be extremely high.