Heck, KotOR helped me fanwank away that "midichlorian" crap from the prequels. I mean, if midichlorians worked the way they were described, why not cultivate them in a lab, and inject them as needed into various Jedi so you could have an army of Mace Windus and Yodas? Why search haphazardly, in an obviously flawed fashion (Tatooine isn't that far off the starlanes), for people who just happen to house midichlorians?
Well, as we can see from the KotOR games, the Old Republic was an incurious society. In four thousand years, there had been no measurable improvements in any of their technologies - blasters, droids, gravity control, and stardrives all worked the same way during the Jedi Civil War and the Rebellion. I hypothesize, therefore, that some sage amongst the Jedi noticed that their trainees tended to have large infestations of midichlorians, and assumed that the midichlorians were precursors to the Force. Later generations took the "received wisdom" of their predecessors, and never came to the (obvious to me) correct conclusion - that midichlorians are in fact a parasitic life form, feeding on the Force potential of a host. Since all sentient life forms tap into the Force to a certain degree (else Obi-wan never should have been able to feel an entire planet die from lightyears away, while in hyperspace no less, in IV), anyone can house at least a minor colony of midichlorians - they appear to cause no negative effects, but are among the harmless parasites. Thus, they can spread across the Galaxy, at the speed of colonial expansion.
By this hypothesis, one wonders if perhaps some Jedi might have become more powerful if they managed to eliminate their own midichlorian infections...
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Sodium is a metal that reacts explosively when exposed to water. Chlorine is a gas that'll kill you dead in moments. Together they make my fries taste good.