Midi-chlorians were intelligent microscopic life-forms that lived symbiotically inside the cells of all living things. When present in sufficient numbers, they could allow their host to detect the pervasive energy field known as the Force. Midi-chlorian counts were linked to potential in the Force, ranging from normal Human levels of 2,500 per cell to the much higher levels of Jedi. The highest known midi-chlorian count—over 20,000 per cell—belonged to the Jedi Anakin Skywalker, who was believed to have been conceived by the midi-chlorians.
Midi-chlorian counts were measured through a blood test; the Jedi used this method to locate Force-sensitive children before their Order was purged by the Galactic Empire. With the rise of the Empire, research into Jedi and the Force was banned, and though midi-chlorians continued to be tested for, sometimes by the Empire itself to root out hidden Jedi and other Force-sensitives, knowledge of them was diminished and inquiries into them were branded as illegal medical research. Midi-chlorians were only rediscovered after the New Jedi Order was founded.
When not forbidden, studies of midi-chlorians occurred among both those who could master the Force and those who could not. While medical teams worked to understand the relationship between midi-chlorians and the Force, Jedi healers performed their own studies of the organisms. Even more esoteric studies were conducted by the Dark Lord of the Sith Darth Plagueis, who discovered a way to manipulate the midi-chlorians to create new life.