A little late here but oh well...
To the points made that a virus or bacteria can kill the whole world, I'm going to touch up on my isolation theory.
First off we have oceans. Mountains. Long stretches of land and water where no one lives. The world is not shoulder to shoulder with neighbors. Every country you can name has at least two bodies of isolated people, whom have very little contact with the outside world. An epidemic could be spreading like wild fire all around them, but they are closed in enough away they aren't affected.
Secondly, at least some countries will close boarders in some manner. Remember all the Asians having to be checked out entering the US and I believe Canada did it even more than we did. Remember how quickly, as soon as 1 case was found in those countries' native soil, so many actions were implicated to avoid more infections?
This isn't the Stand. This isn't a superflu, Captain Trips isn't going to spread like wild fire. It takes time for a viral disease to spread, even if it were highly contagious - super contagious - it would still take time and need a carrier. Certain diseases don't do well in the cold, we have people living in the Arctic, others do poorly in dry areas such as Phoenix or even parts of the Middle East. Countries will shut their boarders, put people into isolation, and simply effectively keep the virus out till it loses enough hosts in the local area to go dormant. If it's a bacteria it gets sampled and wholloped even more so than it would otherwise.
Virus' can live for ever, but they can be for lack of a better word sedated and brain freezed to the point they aren't going to do anything, they can be killed in all effectiveness. Generally this happens in a number of ways, but I'm more speaking environmentally. Ebola can be killed for example, in the fluids and mucus it is hiding in. Bacteria can be treated. Bacteria, generally as a rule of thumb, are often easier to destroy down right than Virus', in a subject.
Plus to say nothing is destroyable, or curable, is a complete wash. Cancer can be considered (while partially incorrect in many cases) curable, even though you still have it in your body. It hides away, no symptoms, and may never come back. You are cured. It may come back, your remission ends, repeats, etc. And in many cases the wording can be used that you are cured. Wording is everything. AIDs would fall under that if you can show absolutely no symptoms for any length of time period, much like cancer.
I mean really, the biggest killer humans have ever known has killed more people than the plague, cancer, and AIDs combined. Shit throw in all the wars we've fought and you might start catching up. Influenza is mankind's deadliest enemy, and to this day it kills thousands. It's the longest running epidemic, there hasn't been a time when hundreds weren't dying from it in heightened numbers, drastically multiplying the local death count, forget flu count, the entire death count.
Some people have such a heightened physiology in a way, they barely ever get the flu. It's not a big deal for them. Me? I get it I soldier on. Friends get it and they're bed ridden for days! I haven't gotten a flu shot in years, and I get it every year an at worse it makes me overly cranky. And yet it's the biggest killer in the world. Shows how horrible most pandemics truly are, in perspective to the modern world.
[/rant]