Marketers cheered in the 1990s when the Internet ushered in an era of unlimited, round-the-clock contact with customers. But cheers turned to jeers when dangerous hackers crashed the party, unleashing harmful computer viruses that threatened the future of e-commerce and the Web. The situation produced two opposite, though equally lucrative, responses: computer security companies set out to beat cybercriminals; advertisers set out to join them. One response produced anti-spam software; the other gave birth to viral Web ads.

Viral ads are those short video clips on the Internet that utilize humor, exclusive content, and gimmicks to promote new products. Like computer viruses, viral ads transmit person-to-person, multiplying their effect through the e-mail boxes of friends and acquaintances. Viral content follows similar guerrilla-marketing efforts from major advertisers like Nike and Microsoft who have sought to generate word-of-mouth buzz through networks of enthusiastic customers.

Advertisers of some of the world's most culturally relevant brands took their products to the masses online in 2006, planning and executing successful viral marketing campaigns. Web users encountered entertaining video clips for The Simpsons, the BMW K 1200 R motorcycle, and Thom Yorke's debut album, "The Eraser."

Yet despite viral marketing's big push in 2006, some observers suggest that the hippest new marketing gimmick may have peaked--at least for now. Video production houses are pricy, and sites like YouTube and Google show over 100 million videos daily and charge hefty ad placement rates. The initial expectation of gaining exponential visibility on the cheap is no longer realistic.

While it may be true that advertisers now get less for their buck, it's hard to imagine that viral advertising won't continue to replicate and mutate—after all, that's what viruses do.