viral marketing
n. The promotion of a service or product by using existing customers to pass along a marketing pitch to friends, family, and colleagues.

Example Citations:
The best viral marketing is not just word-of-mouth, as some people carelessly assume. Nor is it multi-level marketing, where Juan sells to Alice and then gets a cut of whatever Alice sells to Fred. It is when users actively recruit other users, not for pay, but because they benefit from a larger user pool, just as virus DNA benefits from the spread of a virus.
—Esther Dyson, "Second Sight: Melissa is a marketing tool," The Guardian (London, England), April 8, 1999

A recent check of newsgroups shows several postings to “comp.sys.ibm.pc.games” and “alt.food.cocacola” talking about the game and swapping tips and hints. Halloran calls this world-of-mouth buzz “viral marketing” since he hopes it’ll spread that way.
—Frances Katz, “Cherry Coke and its promo game ... it’s the online thing,” The Atlanta Journal and Constitution, June 7, 1998

Earliest Citation:
At Ernst & Whinney, when Macgregor initially put Macintosh SEs up against a set of Compaqs, the staff almost unanimously voted with their feet as long waiting lists developed for use of the Macintoshes. The Compaqs were all but idle. John Bownes of City Bank confirmed this. "It's viral marketing. You get one or two in and they spread throughout the company."
—Tim Carrigan, "New Apples tempt business," PC User, September 27, 1989

Notes:
Why "viral" marketing? Probably because a biological virus replicates itself by invading a host cell and then using the cell's machinery to create new copies of the virus. So, analogously, a customer acts like a kind of "host cell" for a company's marketing message, and that customer is used to create new "copies" of the viral marketing message.

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