Viral marketing works very differently from other forms of classic advertising. A content could go viral by chance or through strategic calculations and planning.
In the case of accidental virality, the creator, which could be the brand owner or not, simply uploads the content with the aim of just sharing. And then it suddenly goes highly popular, much to the surprise of the uploader and delight of the brand owner (if there is a brand included in the viral content).
A good example of accidental virality is in the case of GoPro. A cameraman set up his GoPro camera by a riverbank, hoping to get some close-up shots of a grizzly bear while filming a wildlife documentary for BBC, only to have his camera chewed on by a grizzly.
Amazingly, the camera remained unharmed, and he uploaded the unusual video on his Youtube page. It became highly popular in no time. The company never expected it. They featured it on their own portal, which generated millions of views and significantly promoted their brand name without spending a lot of money on advertising.
In viral content, is not entirely obvious who the sender is and who the intended recipients are. This is very much unlike traditional advertising on TV, radio, and website banners where advertisers are already perceived and audiences are targeted.
Also, it is not something you can easily control.
Since the recipients themselves do the advertising, albeit unknowingly, viral marketing is often perceived as word-of-mouth. As it turns out, such content is more acceptable to the audience since it has been recommended by a friend or a family member. In other words, the audience goes from being passive recipients to active promoters of the advertising content.