The Quickening

While traveling in the Gamma Quadrant, Kira, Dax and Bashir respond to an automated distress call from a planet the Jem'Hadar destroyed two centuries before. The attackers contaminated the inhabitants with a terminal and inheritable disease. Bashir and Dax transport to the planet's surface, where they encounter Trevean, a "healer", who in reality poisons those who are dying in order to end their suffering. Bashir wants to help save them, but Trevean insists that there is no cure.