The "Silver Blood" was a biomimetic lifeform from a Y Class "demon" planet in the Gamma Quadrant. Appearing as metallic liquid in its natural state, this deuterium lifeform had the ability to change itself into that which it came into physical contact with, be it biological or technological.
The Silver Blood was encountered by the crew of the USS Voyager in 2374. While attempting to mine the Silver Blood for deuterium, Tom Paris and Harry Kim came into contact with the substance and were duplicated down to their memories, Starfleet uniforms and combadges, but were unable to survive in a Class M environment. Upon duplicating the Humans, the Silver Blood became sentient for the first time and quickly hungered for more. Holding Voyager physically hostage on the surface of its homeworld, the Silver Blood copy of Kim negotiated to have Captain Kathryn Janeway give over samples of her crew's DNA so that they could be copied. She agreed and Voyager was allowed to leave with a supply of deuterium. (VOY: "Demon")
By 2375, the Silver Blood had replicated Voyager itself, including its computer memory and programs, as well as The Doctor. By this time, the crew was able to breathe oxygen and had forgotten that they were copies of the original Voyager crew. Traveling across the Delta Quadrant while trying to return to Earth, which they thought was home, the crew had invented or acquired an enhanced warp drive. While the effect of this new warp core had no harmful side-effects on cellular life, it did however cause all people and objects made of the Silver Blood to eventually break down and die, striking first the engineering staff and subsequently spreading. Having discovered their true nature, the crew tried to land on another Y class planet, only to be thwarted by the planet's claimants. Setting a course for their original homeworld, the crew made a log buoy detailing their history for others to find from non-Silver Blood components, but it was destroyed in the torpedo tube before it could be launched. The ship and remaining crew literally fell apart in sight of the real USS Voyager; all hands were lost and the real Voyager never knew of the fate or even the existence of its counterpart. (VOY: "Course: Oblivion")