Traditional marketing makes the claim that a product's or idea's ability to succeed or become popular is primarily determined by its quality, price and advertising. However, Berger contends that this is an incomplete picture, as word-of-mouth and social influence play a much larger role in driving "virality" and ultimately account for 20–50% of all purchasing decisions. In fact, “word-of-mouth,” he explains, is effective because it is more persuasive (people trust what others tell them much more than they trust ads they see on TV) and more targeted (people share stories with those who are actually interested in the topic). Contagious describes what makes products, ideas, stories and news likely to become viral through social influence and word-of-mouth, raising brand awareness or growing a customer base. There are six principles: 

1. Social Currency: We share things that make us look good 

People "self-share" their experiences, feelings and ideas to make themselves and their lives seem more exciting and interesting to others. This can be done through posting on social media platforms like Facebook or X or by sharing an entertaining tale with others over social gathering. We employ this kind of word-of-mouth advertising as "social currency”, that allows us to exchange people's perceptions of us. In order to accomplish this, one of the following three things has to be made: a. Find inner remarkability by coming up with something original, strange, unexpected or novel. b. Leverage game mechanic by building something entertaining and engaging by utilising game aspects to draw in players. c. Make people feel like insiders with attractiveness is derived from exclusivity and scarcity. 

2. Triggers: Top of mind, tip of tongue 

While social currency encourages conversation, "triggers" ensure that ideas and products remain fresh in the minds of customers and that they will continue to discuss your business, brand and ideas. People are more likely to interact with and spread the word about your business, brand and ideas if you can get them to think about it (and think about it at the right moment). Frequency (the frequency with which we interact with a trigger) is what makes them effective. 

3. Emotion: When we care, we share

Our emotions force us to act. They elicit laughter, shout, share, converse and purchase. Emotions can motivate behaviour in both positive and negative ways. The degree of arousal we experience with a certain feeling is what motivates us to take action. Positive and negative emotions that are high-arousal motivate us to take action, while negative and positive emotions that are low-arousal keep us inert.

4. Public: Built to show, built to grow 

Brand that are more widely known and easily observed are easier to copy, which increases their likelihood of becoming well-liked and creating enduring memories that endure long after the initial engagement. Berger refers to the idea of using other people's actions as "social proof" to help us overcome our own uncertainty. People follow their eyes, thinking that "monkey see, monkey do". They base their decisions on what they observe. In college, people tend to binge drink because they observe their peers doing the same. 

5. Practical Value: News you can use 

People enjoy being of service to others and feeling helpful. Sharing helpful knowledge that will enable others to save time, energy, and resources is the essence of practical value. You are probably going to tell someone you know about a product, service, cause, or article that has real-world relevance. Furthermore, despite geographical distances, useful ideas and products are shared to benefit others. For this reason, parents frequently give their children helpful articles, cleaning and cooking tips. This helps to maintain social ties even in situations when distance presents challenges.

6. Stories: Information travels under the guise of idle chatter 

The best approach to communicate ideas and information is through stories. Because we are story beings, humans tend to remember and tell stories often. When you tell friends about a terrific deal you found, you will probably tell them about your entire experience. Superfluous details are often overlooked, so to pique buyers' interest, incorporate your product or idea into a narrative that highlights important aspects of your business and includes additional "sticky" elements like humour, quirkiness and originality.