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 counterintuitive, 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.