Viral advertising is creating something worth sharing
Obviously it is a little more complex than that, but it's a start to achieving viral transmission.
Something worth sharing can come in many forms. It can be a call to action, a well thought out advertisement or video, a revolutionary product, or even a rabbit being hauled away by a hawk in front of a family. There is one thing in common with each of these scenarios. They are all worthy of multiple levels of sharing... viral!