i realize i am jumpin' in kinda late here, but...
i am a registered nurse, "no known medical history" is a VERY COMMON and ACCEPTED terminology used by medical providers. it has always meant that the patient/person has not been DIAGNOSED with any medical problems such as diabetes, hypertension, carcinoma, cardiac disease, congenital malformations, metabolic disorders, etc.
(self limiting conditions such as sore throat, common cold, and ear infections are generally not considered a "medical problem" unless chronic and/or affects the patient in a long term manner.)
"no known medical history" does NOT mean the patient has never been seen by a medical provider or imply that the patient has NOT had routine physical examinations.
i have been an RN for 12 years and have worked in hospitals and in private practice. i have seen this term "no known medical history" hundreds if not thousands of times.
again, i apologize for jumpin' in.