They were once a great civilization, with magnificent cities and had the technology to travel to other worlds. Due to their resistance against the Dominion in the mid-22nd century, the Jem'Hadar infected the Teplans with a Blight that was incurable. The disease infected every child while still in the womb, meaning every member of their species was born with the Blight. Most died from it, some as children, others after reaching adulthood. The final stage of the disease was incredibly painful, as the Blight "quickened" and destroyed their body.
After generations of these conditions, their technology had been all but lost, their population was continuing to drop off (as most people did not live long enough to reproduce), and their culture had become obsessed with death and the easing of pain rather than any kind of forward progress. What doctors remained were able to offer little more than relief from the pain of Quickening and a clean, warm bed to die in.
In 2372, Doctor Julian Bashir discovered that the Blight in fact increased its rate of progression when exposed to almost any kind of electromagnetic field (like his advanced medical equipment). Eventually, using less advanced equipment, he was able to develop a vaccine that would prevent transmission of the Blight to future generations by inoculating pregnant women to protect their unborn children. His discovery brought new hope to the Teplans, giving them hope once again that their children and future generations might have a real future other than suffering and death. (DS9: "The Quickening")