"Have you ever thought about yawning, for instance? Yawning is a surprisingly powerful act. Just by reading the two yawns in the previous two sentences--and the two additional yawns in this sentence--a good number of you will probably yawn within the next few minutes. Even as I'm writing this I've yawned twice. If you're reading this in a public place, and you've just yawned, chances are that a good proportion of everyone who saw you yawn is now yawning too, and a good proportion of the people watching the people who watched you yawn are now yawning as well, and on and on, in a ever-widening, yawning circle.
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 audio-tape 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? I suspect that for some of you it did, which means that yawns can also be emotionally contagious. Simply by writing the word, I can plant a feeling in your mind. Can the flu virus do that? Contagiousness, in other words, is an unexpected property of all kinds of things, and we have to remember that if we are to recognize and diagnose epidemic change."