Viral adverts are spread online through emails and blogs
Viral advertising has taken the internet by storm.
From an apparently impromptu dance in Liverpool Street station, to the online campaign by President Barack Obama, the internet is filled with advertising campaigns that seek to be spread by word of mouth rather than traditional broadcast methods.
The chocolate maker Cadbury, for instance, has produced an advertisement which worked through its very incongruity.
Two schoolchildren wiggling their eyebrows to dance music has been a surprise hit - in the same way a film of a drumming gorilla was for the company in 2008.
The advert is proof that big companies are learning how to spread their message using more creative - and subversive - techniques.
Matt Golding, director of viral advertiser Rubber Republic, says that the phenomenon is nothing new.
"The word viral is used because whatever it is you're talking about has been passed from person to person," he explains.
"People have been sharing information and recommending things to each other forever. The internet just allows that to expand."
According to Mr Golding, there are some companies who use the viral strategy to hide the fact they are selling something, but in most cases, viral adverts only work if they are genuinely interesting enough for people to want to pass on.
"If you create something that is entertaining enough you don't need to hide too much behind it," he says.
"It's about offering the audience something. TV advertising is a broadcast system where you pump things at people, this is about making something that people are willing to find for themselves."
Dancing for millions
Someone who knows a thing or two about the power of viral adverting is Matt Harding, a former software developer who left his job to travel the world in 2003.
What troubles me is the blurred line between things that are accidentally interesting and things that are deliberately made
Matt Harding, maker of Where the Hell is Matt
While in Vietnam, one of his friends suggested that he do his trademark funny dance on the streets of Hanoi. He thought the video made a good memento of his travels and took similar videos in the other locations he travelled to.
On his return home he compiled the dancing videos and sent the result to his friends.
"When I made it I didn't think it would be of interest to anyone who wasn't me or someone I knew well," he says.
"But people seemed to take something out of it that I didn't deliberately put in. An expression of joy at getting to see the world. It just seemed to make people happy."
By 2005, the video had been watched by two million people.
He was sponsored by a chewing gum manufacturer and his third video, where he dances in worldwide locations with people who got in touch following the first two videos, has now been released. It has had more than 20 million views and an hourly broadcast on the video screen in New York's Time Square.
For Mr Harding, the idea that viral marketing has forced companies to produce adverts that people actually want to watch has to be seen as a good thing, but the question of authenticity is still a difficult one.
"What troubles me is the blurred line between things that are accidentally interesting and things that are deliberately made.
"You have these companies out there that try to manufacture that and creating essentially fakes. That's an unfortunate trend."
So the Today programme has decided to see if viral advertising works. Mr Golding's company has been challenged to produce a viral ad for the Today programme. "It's a big challenge but we're up for it," he says.
And as Evan Davis says, as soon as we see it back in the Today programme inbox, we'll know it has worked. Watch this space.