Before you string me up, I said "viral marketing program" not "viral marketing." There is a distinct difference. Viral is a collection of tactics, as opposed to a medium. A medium can be a marketing program -- television marketing program, radio marketing program, even internet marketing program -- but a collection of tactics cannot.

I am surprised by the number of marketers I run into who say, "We've created a viral marketing program for our client." I cringe... and wait for the, "we added a 'refer a friend' link to our email." When I explain the inaccuracy, I am not only greeted with skepticism but outright acrimony.

What is viral? Dilution of a term...
All that most marketers mean when they mention "viral" to clients is that they are conscious of the fact that the client's program could reach an acceptable level of self-sustainability by inclusion of elements that encourage re-distribution.

However, what the client hears when agencies say this is, "the internet is going to be buzzing about us because we are doing a viral marketing program!" If your program is just adding an email box and a redistribution method, it is not a viral marketing program, but it is a marketing program that uses viral marketing techniques. You may think I am just on a semantic tirade, but in our field -- one of ever more complex subtleties -- we must be concise and precise.

Physical versus virtual barriers to message adoption
Viral just uses some proxy (often technology) to break down the physical barriers existing between traditional verbal-proximity word-of-mouth geographic pockets.

Huh? Ok, every decent marketer understands that physical presence word of mouth (WOM) is the most trusted form of consumer influence; however, its limits are geographic in nature. What the internet and its distance-skewing communication enabled was the breakdown of many of the physical presence separation barriers.
 
The proliferation of communication -- SMS, IM, email, blogs, chat rooms, et cetera -- facilitated a much faster transference; virtual word of mouth, aka viral, was born. The use of these proxies increases the potential and likelihood that your message will be passed along.

You are looking for Patient Zero
Any virus can be traced back to Patient Zero, the origination point. You can infect someone with your marketing virus, but you cannot guarantee that the self-replicating process will take and turn him or her into a product advocate.

If your timing is right, you have the technology to monitor and shape the conversation and you tend to understand the interconnections of how people communicate better than others, your marketing becomes a meme. Seek to be that meme.