New Yorker writer Malcolm Gladwell looks at why major changes in our society so often happen suddenly and unexpectedly. Ideas, behavior, messages, and products, he argues, often spread like outbreaks of infectious disease. Just as a single sick person can start an epidemic of the flu, so too can a few fare-beaters and graffiti artists fuel a subway crime wave, or a satisfied customer fill the empty tables of a new restaurant. These are social epidemics,...