The Idea Virus

The notion that an idea can become contagious, in precisely the same way that a virus does, is at once common-sensical and deeply counter-intuitive. It is common-sensical because all of us have seen it happen: all of us have had a hit song lodged in our heads, or run out to buy a book, or become infected with a particular idea without really knowing why. It is counter-intuitive, though, because it doesn’t fit with the marketer’s traditional vision of the world.
Advertisers spent the better part of the 20th century trying to control and measure and
manipulate the spread of information—to count the number of eyes and ears that they could reach with a single message. But this notion says that the most successful ideas are those that spread and grow because of the customer’s relationship to other customers—not the marketer’s to the customer.

Its a really cool concept, and I think it applies to much more than just marketing. Read more, or find the whole book online.