Voyager's Doctor is a holographic figure — an emergency medical program devised by Starfleet programmers. When the ship's doctor and entire medical staff were killed in the "Caretaker's" displacement wave, the Doctor by necessity became the resident physician aboard the U.S.S. Voyager, assisted by first Paris
and then Kes
, a quick study in medical training.
The program's first statement upon activation is usually "Please state the nature of the medical emergency;" the automatic command was altered to allow his own creativity, but the Doctor preferred the known opening to creating his own more clever and personable lines. For security's sake in a crisis it carries its own power grid separate from the nominal ship's Holodeck system. His wide array of programming has allowed him to keep Neelix
alive with holographic lungs, save the Vidiian hematologist Danara Pel via a temporary holographic body, and even to alter DNA so as to remerge Torres
' human and Klingon halves, reform Paris
from their retro-evolution as amphibians, and ensure the safety of Wildman's human-Ktarian baby at birth.
From The Official Star Trek Website