The EMH was created by Dr. Lewis Zimmerman, with assistance from Lt. Reginald Barclay, as a short-term emergency medical supplement for use by Starfleet. It possesses one of the Federation's most extensive medical databases, containing information from over 3,000 cultures and the personal experiences of 47 specific surgeons. The program itself is built on an adaptive heuristic matrix, pioneered by Dr. Zimmerman, which allows the EMH programs to learn and adapt quickly to new situations.
Four versions of the EMH series have been created to date, the Mark I, II, III and IV. While the status of the latter three incarnations is unknown, the Mark I programs, based on Dr. Zimmerman himself and possessing his short temper and numerous other character flaws, failed to live up to Star Fleet expectations and was re-assigned to menial labour. Nicknames for the Mark I's include 'Emergency Medical Hotheads' and 'Extremely Marginal Housecalls'.
Only two Mark I's escaped this fate, the EMH's of the USS Voyager and the USS Equinox, both of which were stranded in the Delta Quadrant by the Caretaker. Of these two, only Voyager's EMH, known as the Doctor survived. With a total run-time of approximately 10 years, he is the oldest EMH known to be in existence.
Dr. Zimmerman has also been involved in a project to create a longer-running version of the EMH, the LMH. The status of this project is unknown following the discovery that Dr. Julian Bashir, the man selected to be the template for the LMH, was genetically enhanced.