Obesity is contagious and can spread like a pox from one friend to another, and then another, and then to one more. Both cooperation and selfishness can spread like a virus. Poor sleep and pot smoking are contagious among teens.
Dave Johns:
Not until midcentury did economists, sociologists, and psychologists begin to study contagion with rigor.
One strand of research has examined the spread of relatively simple behaviors: things like coughing, applause, and face-rubbing. Another strand has looked at more complex contagions—speeding, baby-making, and suicide.
A newer area of interest is emotional contagion, which has gotten a boost from the discovery of so-called “mirror neurons”—contagion receptors in the brain that supposedly facilitate the transmission of contagious anxiety, satisfaction and fear.