Why Contagious? You could read Jonah Berger’s Contagious: Why Things Catch On as an update to Malcolm Gladwell’s The Tipping Point: How Little Things Can Make A Big Difference, but I wouldn’t (lol). 

Berger’s assertion that Gladwell is mostly wrong seems moot. The Tipping Point is philosophy. Berger’s book is ditch digging psychology based on his empirical research at Wharton.

Berger shares 3 key ideas to create viral marketing:

• Find or Create Inner Remarkability.
• Leverage Game Mechanics.
• Make People Feel Like Insiders.

This Curatti.com post shares my favorite examples for each of Berger's Contagious traits including Red Bull for "Inner Remarkability", Scoop.it for leveraging game mechanics and Rue La La for making people feel like insiders.