How can the H5N1 virus become a pandemic virus?
The H5N1 virus can improve its ability to infect humans when "reassortment" occurs, resulting in a new virus with enhanced human transmissibility.
This is caused by an exchange of genetic material between human and avian influenza viruses when a human becomes infected with both at the same time.
Pigs may also serve as "mixing vessels" for this exchange and then transmit the new virus to humans. Such an event would be announced by a sudden surge of cases with explosive spread.
A more gradual process of "adaptive mutation" might allow the virus to develop the ability to efficiently bind to human cells.
With subsequent infections to humans, adaptive mutations would appear as small clusters of human cases with some evidence of human-to-human transmission.
This slower mechanism would give the world more time to take defensive action.