Viral marketing is a technique in which one tries to convey his message using pre-existing social network to increase its awareness through self replication.
It is just like a computer virus which spreads by self replicating, this technique uses similar strategy.
It depends on the rate at which information gets passed on to other people. If the pass-on is low then this strategy won’t work.
This can be done using word of mouth strategy in which the message is forwarded on internet, social networking sites, videos, etc.
Eg: One of the best examples of viral marketing is launch of “Kolaveri Di” song on YouTube.