During a decade in Interactive marketing, I’ve heard a litany of long answers to this question. Too many of them assume “viral” only happens on the Internet, or that “viral” means “short online video clip with some immeasurable entertainment value”. The true reason we call it “viral” marketing is because we notice a message behaving in ways analogous to a biological virus.

Biologically (or pathologically) speaking,  “a virus is an infectious agent that is unable to grow or reproduce outside a host. [Viruses] use the machinery and metabolism of a host to produce multiple copies of themselves.” [source] Basically, a virus needs a host to survive and replicate.

For all advertisers, the “host” is the audience. So, consider the audience your only medium. Everything else is a channel of this medium. Television, outdoor, mobile, console games, banner ads, radio, branded USB drives, micro-sites, kiosks, widgets, print, DVDs, podcasts… these are all channels of the audience.

Early adopters of the Internet could see the potential for digital media, particularly the Internet, to revolutionize the way messages spread through an audience.

Harvard Business School’s Jeffrey Rayport coined the term “viral marketing” in a December 1996 article for Fast Company magazine: “Viruses do not spread by chance. They let the high-frequency behaviors of their hosts -- social interaction, email, Websurfing -- carry them into new territories.” [source]

In his 1994 book Media Virus!, Douglas Rushkoff explored the idea of viral marketing, from within the larger idea of a "media virus" (which is essentially a synonym for a meme): “if such an advertisement reaches a 'susceptible' user, that user will become 'infected' and can then go on to infect other susceptible users.”

Take “twoallbeefpattiesspecialsaucelettucecheesepicklesonionsonasesameseedbun” for example:

Consumers have had this run-on Big Mac lyric memorized since its introduction in 1975. It is arguably the most viral message McDonald’s has ever given the English-speaking world, and I’m fairly certain it was neither pitched nor sold as “viral”. Not in 1975.

Which makes me think: perhaps none of our ideas should be pitched or sold as “viral”. Viral is a behavior of the audience, not the ad agency. We can craft a message (in the form of a video, a website, a catch-phrase, or whatever form of “brand experience” we dream up), but we don’t make it viral. What we make are means by which a message can become viral; we help it along, make it “sticky” and “easy to spread”.

To keep things in the parlance of biology, we create the (ideal) conditions through which a virus can become contagious, find adequate hosts, and replicate. To marketers, "viral" is simply another term for word-of-mouth, however enabled by the technology available at the time.

All that said, there is a short and simple answer to the question posed in this post’s title:

For interactive marketers, the comment may be the single most-feared aspect of an online social network. If you don’t allow comments, you risk being seen as anti-social, thus defeating the purpose of your presence on a social network. (The Web is predicated on conversation, after all.) The fear of what people really have to say about us is something we have got to get over, and quickly. There are missed opportunities in comments, when you see comments for what they really are: the online equivalent of gossip.

That’s a conclusion I arrived at after reading today’s BoingBoing post about the science of gossip. Evolutionary psychologists exploring why we gossip share their latest on why we can’t stop ourselves from dishing:

In many popular online social networks, the comment is the currency of socialization. If you post a funny photo to Facebook, I leave a comment on it. If my band posts a new song to MySpace, we know how well it is liked by the comments we receive, and by how many other users add that song to a playlist.

As smaller groups within networks become more established, they also tend to self-regulate. For example, Consumerist.com hired a Comments Moderator who regularly updates the site’s Comments Code – which has actually brought back readers who abandoned the site after experiencing inappropriate comments from other users. The code reassured site members of the accepted norms of this community. Smaller special-interest groups like this, and the measures they take to reinforce a sense community, resemble the workings of tribal culture from which modern civilization evolved.

Translated to online social networks: your brand may be the stranger who’s wandered into a small group with little or no proof of membership. That’s when it’s time to start speaking the language.

If comments are a form of gossip, and
Gossip is proof of membership in a group, then
Comments are proof of membership in a group.

Brands that want to fit in on social networks should take note. Consider leaving some insightful comments out there, and forget what your mother taught you about gossip being all bad.