The viral growth engine is based on viral growth which could be categorised as word-of-mouth.
A sustained, non-booming, viral growth is a great way to reduce marketing costs by letting the customers do the marketing for you!
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Viral growth can be expensive
When one goes truly viral, it could bring in a lot of new users. When the business model is not well validated one might actually incur costs.
Validate that the business model or curb viral growth when this boom might happen.
Reference
Ries, Eric. The Lean Startup: How Today’s Entrepreneurs Use Continuous Innovation to Create Radically Successful Businesses. 1st ed, Crown Business, 2011.
Highlights or timestamps
The viral engine works by getting existing customers to take care of the company’s marketing. Awareness of the product spreads among your target customers by word-of-mouth. This can save you a lot of marketing expenditure, so you should make it as easy as possible for customers to engage in this kind of viral marketing.
— ^e5bb7b from The Lean Startup