It is a pandemic if it affects the entire world.
From my understanding, this is what happens: A chicken gets the Avian flu, and excretes it. Someone inhales it and gets its flu. Now, if humanity has bad luck, this person also has the conventional flu.
The Bird Flu and the Conventional Flu teach each other new tricks and mutate into SuperFlu(tm). SuperFlu(tm) has a very high mortality and it able to infect people just like the regular flu can - contact, air, etc. SuperFlu(tm) originates somewhere (lets say in Asia). A rural farmer in China migrates to Shanghai. Suddenly, hundreds of thousands of people are exposed to SuperFlu(tm). Shanghai is a very large hub for all of Asia.
Businessman is in Shanghai and comes across SuperFlu(tm). After concluding a deal with a Japanese firm, he flies over to his home nation of France. Thousands of businessmen are doing just what he's doing - they return to their places of work and/or home nations. Suddenly SuperFlu(tm) is in France, Germany, Russia, Taiwan, Japan, Korea, Australia...SuperFlu(tm) is spreading all over the entire world. Its mortality rate is 50-75% and with millions upon millions of people carrying it, moving across the globe like insects...50 million suddenly doesn't seem like such a far out number.
It becomes a pandemic.
Of course, if the mortality rate is *extremely* high then it may just burn out - infect quite a few people but they die so quickly they dont get the chance to spread it.
Some could be immune.
The experimental vaccine could work. Yugoslavia once had an outbreak of smallpox (the final outbreak of smallpox) - they declared martial law, locked down everyone in Yugoslavia and innoculated the entire population. If the same were done in a nation that experienced an outbreak, if they did it quickly, it could be stopped in its tracks. Sure, the experimental treatment may not work, but in my opinion it would be far better to take the chance than to lag behind.
It might not develop into SuperFlu(tm) at all - it could mutate but not become superinfectious.
Maybe what we should stop doing is raising billions of chickens for slaughter to be consumed