Force-sensitivity is a condition in the Star Wars universe where a life form possesses a natural connection to the Force. Though the Force flows through all life, with only rare exceptions (such as the Yuuzhan Vong), outright sensitivity to it is a more uncommon trait. While potential for Force-sensitivity is established at birth, awareness, experience and training are necessary to harness the power of the Force. Yoda implies that this training is most effective in early childhood. It is established in The Phantom Menace to be biological, the product of midi-chlorians.
Midi-chlorians are a microorganism first mentioned in Star Wars Episode I: The Phantom Menace. They are microscopic life-forms that reside within the cells of all living things and communicate with the Force. They are symbionts with all other living things and without them life could not exist. The Jedi have learned how to listen to and coordinate the midi-chlorians. While every living being thus has a connection to the Force, one must have a high enough concentration of midi-chlorians in one’s cells in order to be a Jedi or a Sith.
Creator George Lucas says that the midi-chlorians are based on the endosymbiotic theory.