A clone grown from a Lyssarian desert larva injected with a patient's DNA, which matures and dies within about fifteen days and can provide perfectly matched tissue for transplant.
A mimetic simbiot is produced by injecting the DNA of a humanoid into a Lyssarian desert larva, one of the creatures Doctor Phlox keeps in his sickbay menagerie. The larva grows into a copy of the donor that passes through an entire lifetime in roughly fifteen days.
In the Enterprise episode Similitude, Commander Tucker is left comatose by an engine accident and needs a transplant of neural tissue. With the Xindi mission at stake, Captain Archer authorises Phlox to grow a simbiot. The clone, named Sim, grows up aboard and unexpectedly inherits Trip's memories and feelings as well as his body.
The plan becomes a moral crisis when Phlox finds that the operation will kill Sim rather than merely shorten his brief life. Sim considers stealing a shuttlepod, then chooses to go through with the procedure, and is given a funeral with full honours.