Tamar was a Teplan. Along with other members of his race, he suffered from the Blight, a terminal disease. The Jem'Hadar contaminated the inhabitants with the disease that was passed on from generation to generation.
In 2372, Teplan volunteered to help Dr. Julian Bashir in hopes of finding a cure for the disease. Dr. Bashir was unable to find a cure for those like Tamar who had entered into the "quickening", or end stage of the disease. He died that year.
Because of those like Tamar, Bashir was able to find a vaccine that would inoculate the fetuses of pregnant women against the disease. His generation would die, but the next generation would survive, and the Blight would end. (DS9: "The Quickening")