These three characteristics—one, contagiousness; two, the fact that little causes can have big effects; and three, that change happens not gradually but at one dramatic moment —are the same three principles that define how measles moves through a grade school classroom or the flu attacks every winter.
Yawning is incredibly contagious. I made some of you reading this yawn simply by writing the word “yawn.” The people who yawned when they saw you yawn, meanwhile, were infected by the sight of you yawning—which is a second kind of contagion. They might even have yawned if they only heard you yawn, because yawning is also aurally contagious: if you play an audiotape of a yawn to blind people, they’ll yawn too. And finally, if you yawned as you read this, did the thought cross your mind—however unconsciously and fleetingly—that you might be tired?
when a virus spreads through a population, it doubles and doubles again, until it has (figuratively) grown from a single sheet of paper all the way to the sun in fifty steps. As human beings we have a hard time with this kind of progression, because the end result—the effect—seems far out of proportion to the cause. To appreciate the power of epidemics, we have to abandon this expectation about proportionality.
The Tipping Point is the moment of critical mass, the threshold, the boiling point.