Taken from: http://www.fastcompany.com/magazine/135/made-to-stick-getting-your-ideas-to-fly.html

Excerpt from article on viral marketing:

Viral marketing has become a hip, low-cost way to reach a lot of people very quickly — with little effort.

It’s emotional — in fact, if you believe it, it’s terrifying.

There’s another emotional angle: When someone shares this legend with you, they feel like they’re doing a public service. They might believe they’re saving your life. And that’s the second trait of viral ideas.

Rosen calls this a "trigger," and it’s the third trait of a viral idea. A trigger is an environmental reminder to talk about an idea.