Viral marketing and viral advertising refer to marketing techniques that use pre-existing social networks to produce increases in brand awareness or to achieve other marketing objectives (such as product sales) through self-replicating viral processes, analogous to the spread of pathological and computer viruses. It can be word-of-mouth delivered or enhanced by the network effects of the Internet. Viral promotions may take the form of video clips, interactive Flash games, advergames, ebooks, brandable software, images, or even text messages.
The marketing campaign for the 2008 film The Dark Knight combined both online and real-life elements to make it resemble an alternate reality game. Techniques included mass gatherings of Joker fans, scavenger hunts around world, detailed and intricate websites that let fans actually participate in "voting" for political offices in Gotham City, and even a Gotham News Network that has links to other Gotham pages such as Gotham Rail, a Gotham travel agency, and political candidate's pages. The movie also markets heavily off of word of mouth from the thousands of Batman fans.
How can you apply this to the case of Slumdog?