In a short theory Viral Marketing is Cheap able, easy to reach, high energetic and many features includes with Viral marketing which can give the popularity to product/brand. The reason behind all of above is
(1). Social network which exists in market, They easily to makes a platform that marketers can communicates people without any hurdles and people easily understand about brand awareness and this is the way of increase product sales.
(2) It’s always truth, the word "Free" is most powerful in a market and many people wants to take a chance. Most company’s policies under Viral Marketing programs i.e. the free and it always create a attraction of your services or product and provide a bunch of people to you.
(3) Without any effort you can provide the information of your product and services. Like when flu spreads in some people and doctor advices to other that how can you safe from them i.e. always away from people who infected by cough & fever, wash your hands when you apart from them and precaution don't touch your eyes, nose or mouth. Because viruses only spread and easily transmit.