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. ______ It is proved that the new strains of flu usually start to develop in the far east. The most common hosts for influenza virus are ducks, chickens, and pigs, which in Asia often live in close proximity to each other and to humans. Pigs are subject to infection by both bird and human strains of the virus, and individual animals are often simultaneously infected with multiple strains. This creates conditions favoring genetic recombination between strains, producing new combinations of H and N subtypes. The Hong Kong flu, for example, arose from recombination between A (H3N8) [from ducks] and A (H2N2) [from humans]. The new strain of influenza, in this case A (H3N2), then passed back to humans, creating an epidemic.