Anything can be viral. There is a popular misconception that for something digital to “go viral” it must involve sex, nudity or jokes. It is a fact that the second biggest method by which people find websites, after search, is via “word of mouth” – i.e. viral. If you email a link, or tell a friend, that is viral.
The trick is knowing how to seed your campaign and making it attractive enough to be sent on/recommended/forwarded. There are some very good digital agencies and web design companies who specialise in viral and there are many non-specialist agencies with the capability of coming up with very good viral ideas but who may need support producing and ditributing it.
The glamourous end of this market is occupied by the advertising based virals, which often actualy started out as a TV ad and then were put online. This trend is changing towards ads produced specifically for online and there are many agencies outside of the traditional advertising agencies capable of coming up with the goods.
But is not all about TV/video - there is also a significant market for viral games - which can be produced at a lower cost and have a greater shelf life in terms of return visits and plays, but not neccesarily the true viral power to go "global".
See below for examples of both.