From the Mayo Clinic & CDC

[Guest guest]

How long might it take for the bird flu to mutate into a serious

human threat?

That's the question of the hour, and the truth is, no one knows. We

do know a few important things about H5N1, the bird flu subtype that

is the greatest cause for concern. First, H5N1 is particularly

virulent, killing close to 100 percent of susceptible birds and more

than half the people who have been infected. Second, the virus

appears to be spreading, including most recently to Europe; it has

now killed tens of millions of birds in a number of countries.

Third, it seems to be affecting more and more species, including

cats, which usually aren't susceptible to bird flu, and pigs, which

traditionally have been a species that acts as a " mixing bowl " for

viruses transmitted between birds and people. But whether this virus

will ever make the genetic changes needed to infect humans on a mass

scale, or how long that might take — there's just no way of knowing.

http://www.mayoclinic.com/health/bird-flu/ID00032

This from CDC:

Pigs can be infected with both human and avian influenza viruses in

addition to swine influenza viruses. Infected pigs get symptoms

similar to humans, such as cough, fever, and runny nose. Because

pigs are susceptible to avian, human and swine influenza viruses,

they potentially may be infected with influenza viruses from

different species (e.g., ducks and humans) at the same time. If this

happens, it is possible for the genes of these viruses to mix and

create a new virus. For example, if a pig were infected with a human

influenza virus and an avian influenza virus at the same time, the

viruses could mix (reassort) and produce a new virus that had most

of the genes from the human virus, but a hemagglutinin and/or

neuraminidase from the avian virus. The resulting new virus would

likely be able to infect humans and spread from person to person,

but it would have surface proteins (hemagglutinin and/or

neuraminidase) not previously seen in influenza viruses that infect

humans. This type of major change in the influenza A viruses is

known as antigenic shift. Antigenic shift results when a new

influenza A subtype to which most people have little or no immune

protection infects humans. If this new virus causes illness in

people and can be transmitted easily from person to person, an

influenza pandemic can occur.

http://www.cdc.gov/flu/about/fluviruses.htm