Medical malpractice is also called medical negligence.  If a health care provider fails to meet generally accepted standards of care in the treatment of a medical patient, and additional injury or disease results from that failure, a claim may be warranted.   As with any claim for professional negligence (whether doctor, lawyer, accountant, architect, etc.), the applicable "standard of care", and any instance in which it was not followed, must be determined by an expert witness - someone whose education, training and experience gives them the right to offer an opinion on those questions.