The term "viral marketing" is offensive. Call yourself a Viral Marketer and people will take a couple of steps back. "Do they have a vaccine for that yet?" . A sinister thing, the simple virus implies doom, not quite dead yet not fully alive, it exists in that nether genre somewhere between disaster movies and horror flicks.
I admire the virus. It has a way of living in secrecy until it is so numerous that he wins by sheer weight of numbers. It piggybacks on other hosts and uses their resources to increase the tribe. And in the right environment, it grows exponentially. A virus don't even have to mate -- he just replicates, again and again with geometrically increasing power, doubling with each iteration.

What does a virus have to do with marketing?
Viral marketing describes any strategy that encourages individuals to pass on a marketing message to others, creating the potential for exponential growth in the message's exposure and influence.
Like viruses, such strategies take advantage of rapid multiplication to explode the message to thousands, to millions.
Off the Internet, viral marketing has been referred to as "word-of-mouth," "creating a buzz," "leveraging the media," "network marketing." But on the Internet, for better or worse, it's called "viral marketing.

It first came about Hotmail.com Example
The classic example of viral marketing is Hotmail.com, Microsoft's first free Web-based e-mail services.

The strategy was simple:
• Give away free e-mail addresses and services,
• Attach a simple tag at the bottom of every free message sent out: "Get your private, free email at http://www.hotmail.com" and,
• Then stand back while people e-mail to their own network of friends and associates,
• who see the message,
• Sign up for their own free e-mail service, and then
• Propel the message still wider to their own ever-increasing circles of friends and associates.
Like tiny waves spreading ever farther from a single pebble dropped into a pond, a carefully designed viral marketing strategy ripples outward extremely rapidly.
It has since been repeated by every major name on the Internet.

Elements of a Viral Marketing Strategy                

Accept this fact. Some viral marketing strategies work better than others, and few work as well as the simple Hotmail.com strategy because it was the one that started it out. But below are the six basic elements you hope to include in your strategy. A viral marketing strategy need not contain ALL these elements, but the more elements it embraces, the more powerful the results are likely to be.

An effective viral marketing strategy:
1.    Gives away products or services
2.    Provides for effortless transfer to others
3.    Scales easily from small to very large
4.    Exploits common motivations and behaviors
5.    Utilizes existing communication networks
6.    Takes advantage of others' resources

Let's examine at each of these elements briefly.

1. Gives away valuable products or services
"Free" is the most powerful word in a marketer's vocabulary. Most viral marketing programs give away valuable products or services to attract attention. Free e-mail services, free information, free "cool" buttons, free software programs that perform powerful functions but not as much as you get in the "pro" version. A Law of Web Marketing is to Give Before You Sell.
"Cheap" or "inexpensive" may generate a wave of interest, but "free" will usually do it much faster. Viral marketers practice delayed gratification. They may not profit today, or tomorrow, but if they can generate a groundswell of interest from something free, they know they will profit "soon and for the rest of their lives" Patience is essential. Free attracts eyeballs. Eyeballs then see other desirable things that you are selling, and……then you earn money. Eyeballs bring valuable e-mail addresses, advertising revenue, and e-commerce sales opportunities. Give away something…then sell something.

2. Provides for effortless transfer to others
Medical professionals offer sage advice at flu season: stay away from people who cough, wash your hands often, and don't touch your eyes, nose, or mouth. Viruses only spread when they're easy to transmit. The medium that carries your marketing message must be easy to transfer and replicate: e-mail, website, graphic, software download. Viral marketing works famously on the Internet because instant communication has become so easy and inexpensive. Digital format makes copying simple. From a marketing standpoint, you must simplify your marketing message so it can be transmitted easily and without degradation. Short is better. The classic is: "Get your private, free email at http://www.hotmail.com." The message is compelling, compressed, and copied at the bottom of every free e-mail message. Of course Yahoo picked up from where hotmail left off (complacency) and then Google worked their market into Gmail, which has not been able to take over Yahoo's service, remarkably enough. Reason for me to believe that Yahoo is the ultimate force in the market in the end. They own the individual's personality (email address) in the long run. But more about that later.

3. Scales easily from small to very large
To spread like wildfire the transmission method must be rapidly scalable from small to very large. The weakness of the Hotmail model is that a free e-mail service requires its own mailservers to transmit the message. If the strategy is wildly successful, mailservers must be added very quickly or the rapid growth will bog down and die. If the virus multiplies only to kill the host before spreading, nothing is accomplished. So long as you have planned ahead of time how you can add mailservers rapidly you're okay. You must build in scalability to your viral model.

4. Exploits common motivations and behaviors
Clever viral marketing plans take advantage of common human motivations. What proliferated MySpace in the early day? The desire to be cool. Free drives people. So does the hunger to be popular, loved, and understood. The resulting urge to communicate produces millions of websites and billions of e-mail messages. Design a marketing strategy that builds on common motivations and behaviors for its transmission, and you have a winner.

5. Utilizes existing communication networks
Most people are social. Nerdy, basement-dwelling computer science grad students are the exception. Social scientists tell us that each person has a network of 8 to 12 people in their close network of friends, family, and associates. A person's broader network may consist of scores, hundreds, or thousands of people. A waitress, for example, may communicate regularly with hundreds of customers in a given week. Network marketers have long understood the power of these human networks, both the strong, close networks as well as the weaker networked relationships. People on the Internet develop networks of relationships, too (MySpace, Facebook, Friendster, Twitter and a couple of thousand other social sites stand witness for this). They collect e-mail addresses and favorite website URLs. Affiliate programs exploit such networks, as do permission e-mail lists. Learn to place your message into existing communications between people, and you rapidly multiply its dispersion.

6. Takes advantage of others' resources
The most creative viral marketing plans use others' resources to get the word out. Affiliate programs, for example, place text or graphic links on others' websites. Authors who give away free articles, seek to position their articles on others' webpages. A news release can be picked up by hundreds of periodicals and form the basis of articles seen by hundreds of thousands of readers. Now someone else's newsprint or webpage is relaying your marketing message. Someone else's resources are depleted rather than your own. Now that’s a real virus!

One last addition I would recommend is to keep it simple. Some social sites are hard to very hard to navigate. Viral Marketng needs to be an "automatic". Also important is a good analysis of the nihe market you're operating in and finding the viral vehicles for your message. Among the thousands available there are a couple or two perfect for you.