The Emergency Medical Hologram is a holographic physician program installed in Starfleet sickbays to provide care when medical staff are unavailable or overwhelmed.
Dr. Lewis Zimmerman developed the EMH at Jupiter Station, modelling its appearance and voice on himself and giving it a vast medical database. The program was intended for short-term use, but aboard Voyager the Mark I became the ship's permanent doctor after the medical staff were killed during the ship's abduction into the Delta Quadrant.
Running far longer than designed, Voyager's EMH developed a personality, interests and a claim to individuality. A later model, the EMH Mark II, served aboard the experimental Prometheus, where it worked alongside Voyager's Doctor to recover the ship from Romulan hijackers.
Meanwhile, many Mark I units were reassigned to menial labor after being retired from medical service. When the Doctor wrote a holonovel about the treatment of holograms, the work found an audience among retired EMHs working in a mine, a sign of the wider debate about holographic rights.