The idea behind the meme itself must be persuasive to truly be viral, but a memetic need not present an idea to spread.
Consider for a moment, the Crucifix. Possibly the most powerful memetic the world has ever produced--which is no doubt why God chose it as a vehicle for His Truth. It doesn't present or explain any ideas, but all the same, it signifies an entire philosophy and belief system that permeates the globe--at least for the time being.
Memetic warfare has been behind the rise and fall of just about every major civilization and religion, primarily through symbolism. Now, however, it's become more visual and textual--rather than simply visceral--to appropriately synchronize itself with our excessively info-laden society (see also: the Bernie Sanders meme in the OP). I was about to post an article about it in the politics forum before I got sidetracked.
