In 2369, Captain Benjamin Sisko, Lieutenant Commander Jadzia Dax, Chief Miles O'Brien, and Doctor Julian Bashir encountered artificial microbes in the atmosphere of a moon in the Gamma Quadrant were an Ennis/Nol-Ennis penal colony was located. The artificial microbes regenerated those who had died on the moon, becoming a curse to the two warring factions.
After Bashir had studied the artificial microbes aboard the USS Yangtzee Kiang, he determined that once they had regenerated a lifeform from death, that lifeform could never live in any environment except on that moon without suffering immediate death. As a result, Kai Opaka, who had been so revived and did not wish to die again, elected to remain on the moon and work to bring peace to its other inhabitants. (DS9: "Battle Lines")