The pathogen of Telurian plague.
Telurian plague was one of the most dangerous pathogens known in the 22nd century.
Stocks of Telurian plague were stored aboard Cold Station 12 for study. They were stolen by Malik in 2154, who intended to use them as biogenic weapons. (ENT: "Cold Station 12", "The Augments")
By 2368, there was still no cure for the Telurian plague. Doctor Beverly Crusher was curious if one had been found by the 26th century. She asked time-traveler Berlinghoff Rasmussen if he knew whether or not the disease had been cured, but Captain Jean-Luc Picard did not allow him to answer out of respect for the integrity of the timeline. (TNG: "A Matter of Time")