What is viral?

I will never forget this video! I was attending SES Conference in London; strange to believe but that was my first online marketing event. I think it was early in 2011, when I started working for QS. At some point somebody was on the stage talking about the concept of "virality" and what are the best practices to create something viral.. and.. BHAM! He played this video. This video was made in a couple of days if I am not wrong. Do you remember the eruption of the Vulcan in Island? All the attention of the media was there: I remember I was in London and I was supposed to fly to Italy but I couldn't because all the airports were closed due to this black ash cloud! I also remember an amazing blue sky and no clouds :) Anyway.. Inspiredbyiceland had the intuition that that was the right time to promote the tourism in island, so they decide to produce a video: they didn't have time and they didn't have budget. Look at what they were able to create.. and guess what! This was viral