Emergency Medical Hologram

The Emergency Medical Hologram or EMH is a sophisticated holographic program used upon Starfleet starships and facilities in the late 24th century to assist medical staff in times of emergency. The program whose formal name is EMH Program AK-1 Diagnostic and Surgical Subroutine Omega-323 was developed by Dr. Lewis Zimmerman on Jupiter Station.

EMHs are programmed with five million possible treatments from the collective information of 2000 medical references and the experience of 47 individual medical officers, as well has having adaptive programming for situations outside of those in their database. EMHs are only meant to be used a temporary measure to assist existing medical staff, or in extreme cases, to temporarily replace them until a new doctor is assigned.

Mark I
Work on the EMH program began as far back as 2360 which was conducted by Starfleet Medical though information on it was known only to a few individuals.

Zimmerman's initial proposal for an Emergency Medical Hologram was presented in 2365 to Dr. Beverly Crusher.

By the time the holographic matrix was completed in 2370, it had to have official approval by Leonard McCoy.

The Mark I EMH was modelled on Dr. Zimmerman, in both its physical characteristics and personality. This was considered a flaw in the program's design by many Starfleet medical personnel, including Dr. Elizabeth Lense of the USS Lexington (NCC-61832). 

Lense later came to appreciate the Mark I EMH's lack of simulated emotionalism.

Given these reactions, the Mark I was quickly replaced on many ships by newer versions.

The existing Mark I EMHs were reprogrammed for more menial tasks, such as scrubbing plasma conduits and mining dilithium ore.

Several Mark I EMHs continued to serve in their intended roles, including "The Doctor" of the USS Voyager and the EMH on the USS Equinox (NCC-72381): both vessels being stranded in the Delta Quadrant, did not, of course, have access to upgraded versions.

A Mark I EMH also remained on the USS Enterprise (NCC-1701-E) into 2379, as CMO Beverly Crusher preferred it to later versions.

The Soong Foundation discovered that more than 600 EMH Mark I models were put into servitude by Starfleet mining dilithium. Soong Foundation attorney Alyssa Cogley-Shaw sued for an injunction for Starfleet to reveal the status of all of its holographic programs. She claimed this was the slavery of sentient lifeforms, but Starfleet appealed the injunction claiming that the Mark I models are photonic tools.

Later Versions
The EMH Mark II was programmed with new physical characteristics and to be a much more polite and friendly individual. New starship classes such as the were fitted with shipwide holographic projection systems so that EMHs could attend emergencies outside of sickbay.

The EMH Mark III came into general use during the height of the Dominion War, and included stringent security protocols. It also featured an improvement of its personality subroutine over the previous models. However, following the end of the war, these security measures came to be viewed as a hindrance to effective caregiving.

The EMH Mark IV came into use in early 2376, replacing the Mark III. 

The EMH Mark VI usage was unknown though it was still in operation on starships by 2409 as seen on the USS Khitomer. This version had pale, blueish skin and a personality similar to that of the Mark I.

The EMH Mark IX was online by 2380. This version looked like a young, female with strawberry-blonde hair, and could replicate herself as necessary. A Mark IX nicknamed "Nina" was active aboard the Enterprise in mid 2380. 

Although Doctor Crusher had previously mentioned that she had sworn to never use the EMH. 

Nina was often used following heavy combat, especially during the Borg Invasion of 2381. 

The EMH Mark XI was in use by 2381.