DTS or Dead Turtle Syndrome is a serious and often fatal condition of many turtles. Turtles suffer well, and they can get DTS before you know it. A turtle is diagnosed with DTS when its condiiton has (often slowly and unnoticeably) worsened to the point where the turtle is dead, it just doesn't know it yet. Miraculously, some turtles do recover (I have one of these), and some don't (I had one of these, too.)
Seriously, if your turtle shows the slightest signs of infection, take action, because it's probably worse than it seems.
Find out more about Colds, Respiratory Issues, and Pneumonia.
Learn more: Diseases of Amphibians and Reptiles, by Gunther Kohler
(I translated this book from German. :-)