A rule of thumb for categorizing epidemics

This from Karl Smith, a professor of public economics and government at the University of North Carolina-Chapel Hill:

A good rule of thumb – I believe – for epidemics economic, biological or social is this: If it spreads along lines of communication its entropic information. If it travels along major transportation routes its microbial. If it spreads out like a fan, its an arthropod. If its everywhere, all at once, its a molecule.