This is the concept that an idea can become contagious – the same way a virus does. The premise of this is that the most successful ideas are those that spread and grow because of the customer’s relationship to other customers – not the marketer’s messaging or relationship to the customer.

This book has been republished since this version, but this is the PDF of Unleashing the Ideavirus by Seth Godin from 2000. It may be 18 years old, but still is relevant and useful. Shared with permission as noted on the 3rd page. Highly recommended reading.