Most strains of bird flu do not cross species and end up infecting humans. However the H5N1 strain has, in the last decade, been known to infect humans and kill them in 5 out of every 10 cases.  It clearly has surmounted the first of two barriers that must be cleared before a bird flu pandemic can take hold in the human population.  The second barrier will be surmounted when this strain acquires the ability to jump from one human to another.  This may never happen, and hopefully will not.  But based on the history of pandemic flu, it seems more likely that this is just a matter of time. Whether this takes one year or twenty is just impossible to say.  

    In 1997, epidemiologists and public health officials from around the world got their first glimpse of an entirely new variety of human influenza. Known as subtype H5N1, named for the surface proteins which the virus carries, the new strain had only previously been observed in birds.  Ominously, the effect of H5N1 on poultry had earned it the evocative title of "Chicken Ebola."  And when it surfaced in the human population of Hong Kong in 1997 it proved to be almost as deadly.
 

Bird Flu cannot be eradicated, we must learn to live with it
  The natural reservoir for influenza virus is in fact the intestines of water fowl. In particular, ducks harbor the virus, and suffer no ill effects from doing so. Because these migratory birds are always on the move, and because they shed virus in the waterways they inhabit, influenza virus travels widely and is easily transmitted from bird to bird. Because of this, domesticated birds, like the turkey and the chicken, which cross paths with the virus, become infected and harbor the potential to transmit the virus to humans (for example, virus secreted onto the surface of eggs could end up infecting egg handlers-- for this reason, if bird handlers notice any of the following symptoms in their birds they should immediately notify Federal or State animal health officials: sudden death; lack of energy and appetite; decreased egg production; soft-shelled or misshapen eggs; swelling; purple discoloration; nasal discharge; coughing, sneezing; lack of coordination and diarrhea).

Often an intermediate host, the pig, is involved in the cross-species transmission of influenza. When this happens, the result is referred to as Swine Flu. Even so, the source of the virus is generally aquatic birds.

So what's the difference?

Read the devastating history of 1918 bird flu pandemic:

What can we do to prepare?