Run away if someone promises that they can make your video go viral. They can’t. It’s a scam. No agency has the magic formula.
All you can do make a great video and promote it well. If a gazillion people share it, then it went viral. Same as with a song: No one can guarantee you a Top-40 hit. All you can do is record a great song and promote it well.
Viral is the result: It’s what happens when people love your stuff so much that they share it with everyone.
Here’s a hysterical take on the topic from agency John St.: