Stephen King had it right in The Stand. Kill off 99.4% of the human population, and see what happens from there. On second thought, King only had it half-right. He should have stopped the book after the first part... no need to add the bizarre "good vs. evil" plot in the second and third parts.
As far as personal tragedy goes, I think you're all a little insane. Personal tragedy is good when it happens, because it allows you to grow, explore your pain tolerances, find out more about yourself, BUT there is no need to focus on the possibilities. When you focus on the possible tragedies that could befall you... if you're doing it right, you will be paralyzed. The possibilities are infinite, and most of them are so so much worse than a tree falling in front of you as you're driving down the road. I ride a bicycle everywhere. That's my mode of transportation. If I were to think about all the ways that personal disaster could strike while I'm riding, I would never get back on a bike as long as I live. I hit a patch of ice, go down, crack my head open... I'm dead. Some dumbass motorist isn't paying attention when he opens his driver-side door right in front of me, I get the door prize, go through his window... I'm dead, or paralyzed. That branch that could have cracked off the tree in front of your car? It falls in front of me, brings my bike to a complete stop, and I get impaled on tree branch. Those of you who use a car to transport yourself have nothing to worry about. Cars today are so safety-oriented that practically the only way you're going to get seriously injured is through shit luck, or if you drive off a cliff. Personal tragedy isn't going to strike you, so until it does, there's no reason to think about it. When it does strike, you'll have plenty of time to think about it, especially if you live in any country that has socialized health-care (Canada, UK, most of Europe {I think}, Australia? Sorry, Yankees, you're SOL).
In short, if we're looking at macro-disasters, go big or go home. Kill off the global population, or don't bother. If you're rooting for a micro-disaster to happen, stop. If it comes, it will come, and you can obsess all you want about it then.