The Lysserian desert larva was an animal lifeform native to the Lyssarrian homeworld. It secreted a viral suppressant, making it useful for the treatment of minor injuries such as cuts and bruises. It also possessed the ability, when injected with the DNA of another organism, to transform itself into a clone of that organism. These clones—called "mimetic simbiots"—grew and matured at a rapid rate, and died after only fifteen days.
Doctor Phlox kept a Lyssarian desert larva in sickbay aboard the Enterprise (NX-01), and in 2153, used it to create a clone Commander Charles Tucker III (named "Sim") in order to obtain neural tissue needed to save Tucker's life. (ENT episode: "Similitude")