"Historian" is a term used by medical professionals (particularly doctors and nurses) to describe a person's ability to "give a good history," that is, to recall and describe details of one's own or one's family or friends' medically relevant background, injuries or illnesses, lists of drugs prescribed and taken, dates and results of surgical operations and procedures, etc. One is a good or poor historian to the degree he can provide this information accurately and if it is of use to the medical professional. This term has nothing to do with the subject of History per se.