Text 3 [A] We try our best to develop a vaccine. [B] The cause of the flu epidemics arising so often in Asia. [C] The general introduction of "BIRD FLU". [D] The reason of the new strain of flu arising. [E] The diseases caused by viruses threat human-beings. [F] The connections between bird and viruses are complex. ______ The new strain of flu arises not by mutation, but recombination. Viral genes are readily reassorted by genetic recombination, sometimes putting together novel combinations of H and N spikes unrecognizable by human antibodies specific for the old configuration. Viral recombination of this kind seems to have been responsible for the three major flu pandemics (that is, world wide epidemics) that have occurred in this century, by producing drastic shifts in Hand N combinations. The "Killer flu" of 1918, A (HIN1), killed 21 million people. The Asian flu of 1957, A (H2N2), killed over 100,000 Americans. The Hong Kong flu of 1968, A (H3N2), infected 50 million people in the United States alone, of which 70,000 died.