The prerequisite for any mobile app any brand builds, is that it is fun or functional. But even if you succeed in getting either of both right, you're still only halfway through your work.
How will you get your app out there? Preferably massively so? If you trust your app to be that good it will take care of itself and 'go viral', you're rising to the same bait as hordes of marketers on Youtube: the ultimate free lunch. Deep down, anyone knows there is no such thing as a free lunch. Admit. 'Viral' is a happy accident. 'Viral' is the effect, never the cause. That is why 'viral' cannot be repeated reliably.
So what is a poor marketer to do? Is there any way to intentionally infect people with your app? Well, there is. You can hack the iPhone App Store's ranking algorithm. And you can harness the pass-around power of everyday people.