How does the bird flu become contagious to humans?
The avian influenza virus must change. The virus can change to become more contagious among humans in two ways. The first is a reassortment event, in which genetic material is exchanged between human and avian viruses during co-infection of a human or pig. Reassortment could result in a fully contagious pandemic virus. The second way the virus can change is a more gradual process of adaptive mutation, whereby the capability of the virus to bind to human cells increases during subsequent infections of humans. Adaptive mutation, expressed initially as small clusters of human cases with some evidence of human-to-human transmission, would probably give the world some time to take defensive action, if detected sufficiently early.
