Viral meme

From Memetics

A meme becomes viral if it reaches a certain critical mass level of fitness and adaptation to its environment, so much that it becomes self-sustaining and can encompass and spread towards other hosts at an exponential rate.

To achieve a Viral status, one host has to infect on average more than >1 person. There has to be in essence a spread of the meme and thus transference, it can be through a medium.