joke

A highly infectious, memetic, audio-borne, non-physical pathogen.
Symptoms include:
Laughter - Laughing attracts civilians that may be infected.
Happiness - The disease affects endorphin reactors, increasing their production.
Talkativity - The disease inflicts an urge to talk, spreading it further.

Andy: "Hey Ben, I have a joke. What do you call a city that smells good?"
Ben: "What?"
Andy: "Aromea!"
Ben: "Haha! Hey Carl!"
Carl: "Yeah?"
Ben: "What do you call a city that smells good?"
Carl: "What?"
Ben: "Aromea!"

This highlights how a joke spreads.