Years before the Clone Wars, Dooku staged a ship crash in which General Grievous was horribly injured, needing advanced cybernetics just to stay alive. As part of Dooku’s fall to the dark side he craved for more power and one day hoped to overthrow Palpatine using Grievous as the instrument to do it.
To do this he soaked General Grievous’ remaining organic flesh in the midichlorian rich blood of Sifo-Dias, his old friend, believing grievous flesh could be turned force sensitive. Obviously, this process failed horribly and it left Grievous with a burning resentment for Dooku, knowing he would never be the powerful darkside warrior he wanted.