haven't really figured this out completely, your choices place you on a continuum.compared to other popular choices." http://www.slate.com/id/2223285/
I expect any number of variables... To that end: I think we are faced with our own demise as it were, from within. The H1N1, an EMP, and several instances inbetween. As the old saying goes: "prepare for the worst, but, pray for the best"!