“The Quickening” is episode twenty-four of season four of Star Trek: Deep Space Nine.
Major Kira, Dax and Dr. Bashir have taken a runabout into the Gamma Quadrant when they receive a distress call from a planet that says it is under attack. When they get there, the planet is in ruins, with its survivors eking out an existence amongst its destroyed cities. The attack happened many years ago.
The planet in question is on the edge of Dominion space, and tried standing up to them. As a result, the Jem’Hadar attacked and destroyed their civilisation. Then they infected the entire population with a disease, a disease that they are born with and is passed onto their children.
The disease remains dormant until it ‘quickens’ at which point the individual dies slowly and painfully. All that can be done is ease them out as painlessly as possible.
Julian thinks he can help, but the disease is a nasty one that has been specifically designed to resist attempts to cure it.