Memes are a social contagion. They result from rapid, broad filtration through our social network and differ only from something like a joke e-mail in that they mutate and are re-expressed as they spread. They can't change so fast as to become unrecognizable, however, and they appear to have a short half life that might be dependent on the size of the social network. In a lots of ways they really are like viruses but they mutate more quickly and consist of information, rather than biological material.
I know, I didn't really say anything that people probably don't already know, at least intuitively. But I am fascinated by networks and wanted to see if I could at least wrap my head around the question within the framework of social networks.