Virus are never fun but they are effective in spreading themselves throughout the body and the internet.  A viral marketing utilizes techniques that make people want to share and send the marketing piece, a Video, Picture or Story, to everyone they know. 

There are many examples of this that everyone got.  The Cell Phone ringing making popcorn pop, the email stating that if your forward it Microsoft will pay you money and the funny pictures that make you laugh.  These things get the full attention of the viewer and then are often repeated "around the water cooler".   Good TV commercials are the most common starting place for a viral marketing campaign.   Budweiser, Coca-Cola and other commercials that buy placements during the Superbowl telecast are betting that people will be talking and sharing those commercials for months afterwords. 

This sort of brand marketing can raise global awareness of your product or service depending on how you insert your marketing message.   Most of the times these strategy's are good for marketing an advertising company but do little to a company's bottom line.   They are normally called stunts and have a short life span.