The Teplans are a humanoid species from the Gamma Quadrant.
By the 22nd century, the Teplans had built great cities and possessed the techonology to travel to other worlds. All that changed when they attempted to resist the Dominion, believing they would be able to. The Dominion had the Jem'Hadar infect the entire population with a disease that became known as the Quickening. The disease killed many Teplans and was also passed on to their children, although very few Teplans lived long enough to have children anyway.
In 2372, Julian Bashir and Jadzia Dax travelled to the Teplan homeworld, where Bashir became determined to find a cure. Bashir believed he was close to cure, before discovering that the electromagnetic field from his equipment was hastening the disease. While he did not find a cure, Bashir accidentally found a way to inoculate future Teplans against the Quickening, by allowing an antigen to be absorbed by a mother's placenta while a baby was still in the womb. After returning to Deep Space 9, Bashir continued his work to find a cure for those with the blight. (DS9 episode: "The Quickening")