Well, we're going to lose about 25-30%, if not more, of everyone infected. But you can figure only 1 in 10 or so infected (which is about what Spanish Influenza did). Spanish flu really is the ONLY parallel we have. If we're lucky, Avian won't be worse. It's certainly a stronger, more advanced, virus. But the mutations that make it more virulant amongst humans MIGHT make it somewhat less deadly.
Now, if it manages to infect the entire human race, yes, it would kill billions.
