"The buzzwords viral marketing and viral advertising refer to marketing techniques that use pre-existing social networks to produce increases in brand awareness or to achieve other marketing objectives (such as product sales) 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 promotions may take the form of video clips, interactive Flash games, advergames, ebooks, brandable software, images, or even text messages. The basic form of viral marketing is not infinitely sustainable.

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" has also been used pejoratively to refer to stealth marketing campaigns—the unscrupulous use of astroturfing on-line combined with undermarket advertising in shopping centers to create the impression of spontaneous word of mouth enthusiasm."

The term Viral Marketing was coined by a Harvard Business School graduate, Tim Draper. The term was later 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., accept the idea) and will then go on to share the idea with others "infecting them," in the viral analogy's terms. As long as each infected user shares the idea with more than one susceptible user on average (i.e., the basic reproductive rate is greater than one - the standard in epidemiology for qualifying something as an epidemic), the number of infected users will grow according to a logistic curve, whose initial segment appears exponential. Of course, the marketing campaign may be wildly successful even if the rate at which things are spread isn't of epidemic proportions, if this user-to-user sharing is sustained by other forms of marketing communications, such as public relations or advertising.

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. In 2004 the concept of Alpha User was released to indicate that it had become now possible to technically isolate the focal point members of any viral campaign, the "hubs" who are most influential. Alpha Users can today be isolated and identified, and even targeted for viral advertising purposes most accurately in mobile phone networks, as mobile phones are so personal.

In response to its use, many sites have started up trying to describe what viral marketing is.

Notable examples

• The Ponzi scheme and related investment Pyramid schemes, are an early and unfortunate example of viral marketing. In each round, investors are paid interest from the principal deposits of later investors. Early investors are so enthusiastic that they recruit their friends resulting in exponential growth until the pool of available investors is tapped out and the scheme collapses.

• Multi-level marketing popularized in the 1960s and '70s (not to be confused with Ponzi schemes) is essentially a form of viral marketing in which representatives gain income through marketing products through their circle of influence and give their friends a chance to market products similarly. When successful, the strategy creates an exponentially growing network of representatives and greatly enriches adopters. Examples include Amway and Mary Kay Cosmetics among many others.

• Early in its existence (perhaps between 1988 and 1992), the television show Mystery Science Theater 3000 had limited distribution. The producers encouraged viewers to makes copies of the show on video tapes and give them to friends in order to expand viewership and increase demand for the fledgling Comedy Central network. During this period the closing credits included the words "Keep circulating the tapes!"

• 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.

• In 2001, BusinessWeek 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.

• Launched in 2002, BMW Films was among the earliest viral marketing campaigns. It attracted nearly 55 million viewers and helped to elevate the career of Clive Owen.

• Burger King's The Subservient Chicken campaign, running from 2004 until 2007, was cited in Wired magazine as a striking example of viral or word-of-mouth marketing.

• Cadbury's Dairy Milk 2007 Gorilla advertising campaign was heavily popularised on YouTube and Facebook.

• The release of the 2007 concept album Year Zero by Nine Inch Nails involved a viral marketing campaign, including the band leaving USB drives at concerts during NIN's 2007 European Tour. This was followed up with a series of interlinked websites revealing clues and information about the dystopian future in which the album is set.

• In 2007, World Wrestling Entertainment promoted the return of Chris Jericho with a viral marketing campaign using 15-second cryptic binary code videos. The videos contained hidden messages and biblical links related to Jericho, although speculation existed throughout WWE fans over who the campaign targeted. The text "Save Us" and "2nd Coming" were most prominent in the videos. The campaign spread throughout the internet with numerous websites, though no longer operational, featuring hidden messages and biblical links to further hint at Jericho's return.

• In 2007 the New York Times' advertising columnist Stuart Elliott wrote about a business-to-business viral campaign for a software company, showing that viral advertising has application in areas outside of consumer marketing.

• The 2008 film Cloverfield was first publicized with a teaser trailer that did not advertise the film's title, only its release date: "01·18·08." Elements of the viral marketing campaign included MySpace pages created for fictional characters and websites created for fictional companies alluded to in the film.

• The Big Word Project, launched in 2008, aims to redefine the Oxford English Dictionary by allowing people to submit their website as the definition of their chosen word. The viral marketing project, created to fund two Masters students' educations, attracted the attention of bloggers worldwide, being featured on Daring Fireball and Wired Magazine.

• The marketing campaign for the 2008 film The Dark Knight combined both online and real-life elements to make it resemble an alternate reality game. Techniques included mass gatherings of Joker fans, scavenger hunts around world, detailed and intricate websites that let fans actually participate in "voting" for political offices in Gotham City, and even a Gotham News Network that has links to other Gotham pages such as Gotham Rail, a Gotham travel agency, and political candidate's pages. The movie also markets heavily off of word of mouth from the thousands of Batman fans.

• Both the second and third games in the Halo series were preceded with viral marketing in the form of an alternate reality game.

Viral expansion loop

A viral expansion loop is similar to viral marketing with one notable difference: viral marketing can't be replicated indefinitely, while a viral expansion loop must be in order for it to exist. When properly conceived and implemented, a viral loop almost guarantees self-replicating growth. Companies that have attempted to utilize viral loops to their advantage include social networking engine Ning, and viral loops power many Web 2.0 icons, including Twitter, Orkut, YouTube, Facebook, MySpace, M3mob.com, Digg and Flickr.