While it isn't as "rewarding" to think and write about, the greatest probability of the SHTF comes from a global pandemic.

Experts in the field state that it is a matter of when, not if. We are now 38 years removed from the last pandemic and while it is not scientifically accurate to state that we are "overdue", it is easy to understand why the MSM uses this term because it encapsulates the more complex predictive models in an easy to understand manner.

Now a global pandemic in and of itself wouldn't necessarily make the SHTF, but it certainly has the highest probability. Consider the H5N1. It has a lethality rate of 50%. Yes, you read that right. 50% of all people that catch this nasty bug will die. The 1918 pandemic had an infection rate of 30-40% and with our far more connected world, 30-40% would be a very conservative infection rate. This has all of the ingredients for the SHTF.

These are the a priori factors and the more difficult question to answer is the probability of H5N1 mutating so that it makes the leap from animal to human transmission to human to human transmission. If this happens (and the influenza virus is one of the most aggressive at mutating rapidly) then all bets are off as to how bad it could get. What worries me is that H5N1 is very prevalent in pigs and China have increased their pig population by several orders of magnitude in the last couple of decades. This is a literal breeding ground for H5N1 to mutate into our nightmare.

Economic collapse, WWIII, etc. appeals to our need to be able to "do something" and a global pandemic is far harder to "fight against", but my personal opinion is that this is the one to really worry about. I don't want to seem to dismiss the others as being impossible, just less likely which was the OP question.

As for preparations. In the event we are faced with this situation, the best defense is to quarantine yourself with NO contact with the outside world. No going to the grocery store, no going to work, no going to the gas station, etc. This also means that you must be prepared to deal with utilities being disrupted, because it is a very real possibility that key personnel will become ill or worse.

1. Economic collapse precipitated by cyber-attacks
Like the stuxnet virus that US/Israeli intel unleashed on Iran's nuke fuels refining systems; but with many states and terrorist orgs getting into the act, until someone creates a software bug that takes out a whole system; for instance the computerized pipeline network that delivers crude oil from the US west to refineries near major cities. Or the Army Corps of Engineers software that controls the locks and floodgates on the Mississippi River system. Or the GPS network. Or the software that controls water levels in the cooling ponds for the spent fuel at every nuclear power plant in the nation.

2. Epidemic

3. Coronal Mass Ejection

4. Biological / Dirty Nuke Terror event

5. EMP attack

6. Market Collapse due to the excesses of capitalism

7. World War

Preparations for the various scenarios overlap. I am prepared enough that I take a few hours off every few days in order to post my pointless warnings on boards like this one.

I believe an economic collaspe or virus... Or war... And nukes.

But mainly the economic collaspe. My plan is BOB, find my friends (since we made a plan), and RUN.

Economic collapse is the most probable scenario which will then lead to martial law, social breakdown and general WROL. My wife and I live very close to a Wal-Mart/Sams Club complex and we are stashing cash to purchase basic supplies the moment it happens. I have already started to keep extra supplies in our dwelling and I make sure to buy gear that can be used for camping(water bottles, filtration systems, knives,etc) which will also come in handy in the event social order breaks down. I have talked to a few friends of ours in the neighborhood about this situation but I don't think they are as worried about it as I am but I know that if and when the time comes, we would band together.

actually i believe solar mass exjections and the best way to prep is to keep things around that dont need electric (practical things you will use ) crank flash lights solor panels (if still working ) yes battrie operated things are good but what happens when the battries run out ? the the regular food water protection and first aid .