To me the key is Logistical Support. If the infection rate and death rate rises to a point in which it over whelms the the systems Medical/Logistical support capabilities then the ‘genie’ is truly out of the bottle.
A truly serious as well as unknown factor will be people reactions and how much of government services, manpower and logistical support might have to be diverted to handle panic arising from such.
We are seeing this in Africa right now if it reaches a point in which it becomes unsafe or impossible for medical teams securely preform their duties the spread of the disease could quick start spreading virtually unchecked.
It would not take long, for instance, for the sanitation workers to figure out they could be getting exposed to the disease and just not show up for work. We have seen how well a garbage worker's strike works in an urban area.
That, however, is just one of many sectors where employees might decide the paycheck is no longer worth the risk, and the absence of or undermanning of those sectors could lead to a breakdown of common services and a loss of civility which has the potential to spread far beyond areas directly affected by the disease, especially with the current social climate in the US having been whipped to a frenzy by recent events and the media.
I think the same quandry applies here as there: at what point do those who enforce a quarantine kill those seeking to escape it out of fear for their lives?
The very real possibilities get very ugly very fast as food, fuel, and other shipments are delayed or stop due to travel restrictions, hazards, or no-go areas.
The system is more fragile than many believe.