Viral marketing relies on people sharing content organically via social networks to reach a large audience in a cost-effective manner. It works best when the content tells an engaging story that elicits an emotional response and encourages sharing. To maximize viral spread, campaigns should create unexpected or surprising content, make sequels to extend the effect over time, and engage viewers to become ambassadors who generate their own related content. The success of viral marketing campaigns can be measured using key performance indicators related to engagement, activity, volume, and economic value.