What if ideas were viruses?
Consider the T-phage virus. A T-phage cannot replicate itself; it reproduces by hijacking the DNA of a bacterium, forcing its host to make millions of copies of the phage. Similarly, an idea can parasitically infect your mind and alter your behavior, causing you to want to tell your friends about the idea, thus exposing them to the idea-virus. Any idea which does this is called a "meme" (pronounced `meem').
Unlike a virus, which is encoded in DNA molecules, a meme is nothing more than a pattern of information, one that happens to have evolved a form which induces people to repeat that pattern. Typical memes include individual slogans, ideas, catch-phrases, melodies, icons, inventions, and fashions. It may sound a bit sinister, this idea that people are hosts for mind-altering strings of symbols, but in fact this is what human culture is all about.
As a species, we have co-evolved with our memes. Imagine a group of early Homo Sapiens in the Late Pleistocene epoch. They've recently arrived with the latest high-tech hand axes and are trying to show their Homo Erectus neighbours how to make them. Those who can't get their heads around the new meme will be at a disadvantage and will be out-evolved by their smarter cousins.
Meanwhile, the memes themselves are evolving, just as in the game of "Telephone" (where a message is whispered from person to person, being slightly mis-replicated each time). Selection favors the memes which are easiest to understand, to remember, and to communicate to others. Garbled versions of a useful meme would presumably be selected out.
So, in theory at least, the ability to understand and communicate complex memes is a survival trait, and natural selection should favor those who aren't too conservative to understand new memes. Or does it? In practice, some people are going to be all too ready to commit any new meme that comes along, even if it should turn out to be deadly nonsense, like: "Jump off a cliff and the gods will make you fly."
Such memes do evolve, generated by crazy people, or through mis-replication. Notice, though, that this meme might have a lot of appeal. The idea of magical flight is so tantalizing -- maybe, if I truly believed, I just might leap off the cliff and...
This is a vital point: people try to infect each other with those memes which they find most appealing, regardless of the memes' objective value or truth. Further, the carrier of the cliff-jumping meme might never actually take the plunge; they may spend the rest of their long lives infecting other people with the meme, inducing millions of gullible fools to leap to their deaths. Historically, this sort of thing is happening all the time.
Whether memes can be considered true "life forms" or not is a topic of some debate, but this is irrelevant: they behave in a way similar to life forms, allowing us to combine the analytical techniques of epidemiology, evolutionary science, immunology, linguistics, and semiotics, into an effective system known as "memetics." Rather than debate the inherent "truth" or lack of "truth" of an idea, memetics is largely concerned with how that idea gets itself replicated.
Memetics is vital to the understanding of cults, ideologies, and marketing campaigns of all kinds, and it can help to provide immunity from dangerous information-contagions. You should be aware, for instance, that you just been exposed to the Meta-meme, the meme about memes...
Text by Glenn Grant, Memeticist who also provides this Memetic Lexicon. Another good article to read is Memetics: The Nascent Science of Ideas and Their Transmission by J. Peter Vajk