Hmm...good question. When I think about it, they all sound like possibilities. Humanity has a history of wars, so I could never exclude that we destroy one another that way. An asteroid has already hit the Earth, and when it did, it wiped out nearly all life-forms that inhabited this place. Who is to say that can't happen again? An infection, I doubt one could spread around the entire globe and deliver a 100% mortality rate, but I can't say that it's impossible either. Over population, I don't think that'll cause our doom. It could cause problems, for sure. But not the end of the world. Basically, what I'm saying is that those are all possible candidates, and if one doesn't happen, then one of the others will...eventually.
Ive always pictured the end of the world being a shower of nukes, but then again, i play way too much Fallout.
Global Warming [Another Ice Age]
One of the only things we wouldn't be able to fight off or prevent.
However I think over-population will be our *first issue, just not the one that kills us off.
What ever happened to the scenario *where some stupid scientest creating a SuperComputer that can learn and eventually takes over the world by creating robot soldiers?*
My money's on over population/over consumption of resources.
Gamma Ray Burst (wikipage)