PARIS A patient who was a frequent blood donor is the eighth confirmed case of the human form of mad cow disease in France, the Health Ministry has announced, setting off new concerns Friday about potential large-scale exposure to the mysterious illness. . Between 1993 and 2003, blood from the patient was used both for transfusions and to make medicines, which were given to thousands of patients, health officials said. All were at least potentially exposing them to mad cow disease, whose route of transmission is still poorly understood, scientists say.