What is the greatest threat to human civilization?
In short, what do you think is the greatest threat to human civilization?
Anything from nuclear war, to global warming to alien invasion will be accepted as an answer. Note that actually causing humanity to go extinct in the process is not a requirement.
I would however prefer concrete answers, not abstracts like "lack of wisdom".
I'd have to say plague myself, though whether artificial or natural, I wouldn't know.
We're more interconnected, and travel faster, easier and more prevalent then ever before, and the rapid response and forceful quarantine infrastructure and personnel that would be required to prevent it are simply not in place.
It doesn't help that the vast majority of nations today are too slow to respond to emergencies and/or too softhearted to properly enforce a proper quarantine in the early stages when its still effective.
Seeing as I have some characters left, let me list off other common answers and why I don't think they're as dangerous:
--Global nuclear war simply doesn't seem very likely since the end of the Cold War and the gradual stripping down of arsenals. Localized cases like the India/Pakistan standoff are still there, but they don't threaten global civilization as a whole.
--climate change, man made or otherwise, simply isn't as apocalyptic as people make it out to be. Human civilization has existed for more then 5000 years in a wide range of fluctuating climatic conditions, and in many ways, we've actually grown more robust to their sudden change, not more vulnerable. Don't get me wrong, there is room for tremendous damage, but its not at "deathblow to civilization" levels.
--Zombies would be quickly wiped out by the military. For an army used to fighting against people with guns, zombies would be something of a vacation.
But back to being serious, what do you think is the greatest threat to human civilization, and why?