I think the "Financial calamity leading to an economic breakdown" option is most likely, but an "Uncontainable bio hazard" isn't really very far behind, IMO. With modern-day air travel, any serious illness with a long active-transmission time span between infection and showing significant symptoms could spread faster than a wildfire on a windy day. It's not that it is truly uncontainable; the problem is recognizing you have a problem, and taking drastic action early enough to contain it. Given that any serious containment actions would interrupt deliveries of food and other critical necessities to major cities, and most cities only have about three days worth of food for the population, I don't think our elected leaders would have the 'nads to make the tough call(s) necessary to halt the spread quickly enough. If it started somewhere else first, and we had a couple days/weeks of warning, then maybe the correct arguments would be made and actions taken, but if it started here or spread here quickly, then many of us would be toast.
"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)