>>3192>super-massive asteroid impact
Well, tracking and deflection. Not a high priority right now, I don't think. But that's about all there is to do.>global thermonuclear war
Wars involve authorities, so they must be safe for humans, at least.>a Yellowstone super-volcano explosion,
That depends on how random that will be. With enough warning, perhaps evacuate. Otherwise, well, deal with the crater.>a natural or biotechnologically engineered super-plague
Read "The Next Pandemic" awhile ago. It's complicated, so far misinformation spreads well in America. When authorities crack down too hard, people evade doctors and quarantine -- most of that comes from other countries. The CDC does exercises, but if you really wanted to prepare, you'd want to increase trust, awareness, and practice with people generally.>a self-replicating nanobot mishap, etc.
I don't know much about that. Be careful, I guess?>>3194>alien species introduces a virus humanity has no way to combat.
It's possible. I don't know if there are a lot of viruses in our world capable of infecting more than a few species, so crossing the bridge from one life system to another seems unlikely. But maybe if I think more along the lines of an invasive species taking over Earth, that seems possible.