The pathogen of Anchilles Fever.
Anchilles fever is a highly contagious and virulent disease, capable of causing deaths numbering in the millions. The vaccine for the disease is extremely rare, and not able to be readily synthesized.
In 2154, a sample of this disease's pathogen was stored at Cold Station 12 for study. (ENT: "Cold Station 12")
In 2364, the planet Styris IV suffered from a vicious outbreak of the disease. The only nearby source for the vaccine was Ligon II, which, after a brief negotiation, gave the USS Enterprise-D the supplies needed to cure the outbreak. (TNG: "Code of Honor")