Are contagious jokes random?

The Huffington Post Contagious Festival is an interesting competition. Remember all of those joke emails/links that are lifted from obscurity to almost global popularity for a week or two via word of mouth (or word of email)? The Post are holding a competition to see if someone can create such a phonemenon, or can this only happen at random? Some of the epidemics I remember from the past are This Land Is Your Land, Troops, Tourist Guy (9/11 photo) and the original episode of South Park.