Viral marketing and viral advertising refer to marketing techniques that use pre-existing social networks to produce increases in brand awareness, through self-replicating viral processes, analogous to the spread of pathological and computer viruses. It can be word-of-mouth delivered or enhanced by the network effects of the Internet. Viral marketing is a marketing phenomenon that facilitates and encourages people to pass along a marketing message voluntarily. Viral promotions may take the form of funny video clips, interactive Flash games, advergames, images, or even text messages.

It is claimed that a satisfied customer tells an average of three people about a product or service he/she likes, and eleven people about a product or service which he/she did not like. Viral marketing is based on this natural human behaviour.

The goal of marketers interested in creating successful viral marketing programs is to identify individuals with high Social Networking Potential (SNP) and create Viral Messages that appeal to this segment of the population and have a high probability of being passed along.

The term "viral marketing" is also sometimes used pejoratively to refer to stealth marketing campaigns--the use of varied kinds of astroturfing both online and offline to create the impression of spontaneous word of mouth enthusiasm.

History
The term Viral Marketing was coined by a Harvard Business School professor, Jeffrey F. Rayport, in a December 1996 article for Fast Company The Virus of Marketing. The term was further popularized by Tim Draper and Steve Jurvetson of the venture capital firm Draper Fisher Jurvetson in 1997 to describe Hotmail's e-mail practice of appending advertising for itself in outgoing mail from their users.

Among the first to write about viral marketing on the Internet was media critic Douglas Rushkoff in his 1994 book Media Virus. The assumption is that if such an advertisement reaches a "susceptible" user, that user will become "infected" (i.e., sign up for an account) and can then go on to infect other susceptible users. As long as each infected user sends mail to more than one susceptible user on average (i.e., the basic reproductive rate is greater than one), standard in epidemiology imply that the number of infected users will grow according to a logistic curve, whose initial segment appears exponential.

Among the first to write about algorithms designed to identify people with high Social Networking Potential is Bob Gerstley in Advertising Research is Changing. Gerstley uses SNP algorithms in quantitative marketing research to help marketers maximize the effectiveness of viral marketing campaigns.

Notable examples of viral marketing
* BusinessWeek (2001) described web-based campaigns for Hotmail (1996) and The Blair Witch Project (1999) as striking examples of viral marketing, but warned of some dangers for imitation marketers.
* Burger King's The Subservient Chicken campaign was cited in Wired as a striking example of viral or word-of-mouth marketing.
* In 2000, Slate described TiVo's unpublicized gambit of giving free TiVo's to web-savvy enthusiasts to create "viral" word of mouth, pointing out that a viral campaign differs from a publicity stunt.
* With the emergence of Web 2.0, mostly all web startups like facebook.com, youtube.com, collabotrade.com, myspace.com, and digg.com have made good use of Viral Marketing by merging it with the social networking.