What causes outbreaks of various kinds? Karl Smith offers up a cheap and cheerful maxim:
A good rule of thumb — I believe — for epidemics economic, biological or social is this: If it spreads along lines of communication it's entropic information. If it travels along major transportation routes it's microbial. If it spreads out like a fan, it's an arthropod. If it's everywhere, all at once, it's a molecule.
Something tells me that Karl has a lot of these mental shortcuts squirreled away in his brain. I think he should collect them in a single place so that we can all vote for the winners and losers.