You might be tempted to think this is just a matter of random luck but it’s usually not. According to Dr. Jonah Berger, Wharton School of Business, Products or ideas become contagious whenever and wherever six key "STEPPS" principles come into play:

S – Social Currency – You share those things which make you look good
You have social currency when you know about cool things which others are not yet
aware of. To get people talking, you have to figure out what is truly remarkable about your
product or idea and what you can do to make people feel like insiders.

T – Triggers – Whatever is top of mind is also at the tip of your tongue
Triggers prompt people to think about your product or idea and to talk about it. Find ways
to design products and ideas which are frequently triggered by cues in the everyday
environment. The more often people think about you, the more they will talk about you.

E – Emotion – When you care about something, you share
If you make people feel something, they will remember better. You can’t help sharing
something which is highly emotional – it’s what humans do. Make sure you pick the right
emotions to evoke and then build in clues which relate back to those emotions.

P – Public – If you build to show, you also build to grow
If people see someone using your product or idea, they’re more likely to want it for
themselves. Find ways to make your idea observable and you have something which not
only advertises itself but also creates a behavioral residue effect.

P – Practical Value – You like having news you can use
People like to help others. Therefore, if you have an idea that is a genuine benefit, they will
spread the word. Highlight the true value of your product or idea and then package your
knowledge and expertise so others can pass it on.

S – Stories – Information travels under the guise of idle chatter
People don’t share information – they tell stories. If you can wrap your idea into the
broader narrative of a story people want to share, they will spread the word. The trick is to
make your message so integral people can’t tell the story without it.

If you’re trying to make a product or an idea become contagious, find ways to build these six "STEPPS"
principles into the design of the product or into the messaging which arises around your product or
idea.  If you can do that, you can make your product go viral.