God I remember reading the Stand- from what I remember the ending seemed really shitty, or might just have been my age at the time w/e.
Just reading back through the topic, I notice that Ygg (who seems to be an angry person it seems) keeps repeating "the black death only killed 3/4 of EUROPEANS" which, by his logic, prevents anything ever arising again. The fact is, the Black Death was not always fatal, which meant it could not wipe us out. There are other diseases, though, which will ALWAYS kill. More than this, modern transport and globalisation means that an outbreak in one place in the world nigh on guarantees it will appear at least once in each part of the world. Look at SARs - that broke out in South America, France, South Africa, even with all the embargos in place.
I expect there will not be a single pandemic, but a cluster effect. A major disease will hit the world which is extremely contagious and airborne and everyone will eventually get it. Do they all die? Well with one that strong, its highly possible, however such a powerful disease needn't kill you- the effect that would have would weaken you to a point where anything else would kill you.
Think AIDs and you have the idea. So, for future reference, we don't want to go fucking any highly contagious monkeys as we may end up with a new mutation.