Constance explains the passive voice clearly. She gives some examples of its abuse, such as this politicians’ favourite — cleverly labelled the ‘past exonerative’.
But Constance doesn’t jump on the everything-active bandwagon. Sometimes sentences are passive for a reason. Maybe the ‘agent’ in the sentence isn’t as important as the subject, or isn’t known. If your car has been stolen, you may never find a specific person to be angry at.
So don’t overuse the passive voice, but don’t fear it. After all, it might be just what was ordered by the doctor.