engineered idea viruses

Scott Adams (creator of Dilbert):

Ideas are a lot like viruses. Neither a virus nor an idea is alive, technically, but both reproduce though contact with other people. And both are hard to eradicate. For example, 20% of the American population believes Obama is a Muslim. That's actually an increase since he was inaugurated.

Most idea viruses are the bad type. But I see no reason we couldn't engineer good idea viruses. Such a virus would have three traits:

1. It must be catchy, so you never forget it.
2. It must be something you are inclined to share.
3. It must cause a positive change in the world.