Its epidermal layer secreted a viral suppressant that was useful for treating cuts and bruises. It also had a more obscure property: if it was injected with DNA from another organism, the larva rapidly transformed itself into a clone of that organism that grew, matured, and died in 15 days. These clones were called "mimetic simbiots" and were practically identical to the original organism, including, in the case of Humans, having its memories.
Doctor Phlox kept a Lyssarian desert larva aboard the Enterprise, which he used in 2153 to create a clone Commander Charles Tucker III. He then extracted neural tissue from the clone, named "Sim", to save the life of the original Tucker. (ENT: "Similitude")