Midichlorians are not the source of the Force, but they sustain themselves from the Force and need a host to do so. They are otherwise harmless, but provide a good indicator for the strength of the Force in an individual. [/fanwank]
I wonder if "Midichlorians" are an allusion to mitochondria. IIRC, the latter are basically the energy production in our cells. So I suppose Midichlorians are producing the "force" just like mitochondria create energy.
Of course, the whole thing is... stupid if we take Obi-Wan and Yodas descriptions from Episode IV-VI. If the force is a field that's in the entire galaxy (and possibly beyond?) if the force requires midichlorians to exist? Or are midichlorians just "force muscles"? But then, how can something like force spirits exist?
Well, I suppose the idea is not _that_ bad if you take it all together. Let's assume the force is literally some kind of force (maybe something like the Higgs field?) in the universe. Midichlorians allow people to interact with it and shape it (but also be affected by it). A force spirit requires someone to master the force strong enough to shape a pattern in the force that persists even after his bodies and all the contained midichlorians die. (Unless we assume that the spirit actually is the midichlorians coming free from the body...)
This still keeps some of the mystery - what is this field. How can it affect us? Why does it allow us to predict the future, how can it shape our minds? Why does it make such strong distinction between the "Dark Side" and the "Light Side" (assuming that's not entirely philosophical).
