The Dragons' Plague struck Pern in the last Turn of the Second Interval and the first of the Third Pass (507-508 AL). It was a disease of the upper respiratory track affecting dragons and fire-lizards, but not whers. The disease itself was fatal, but most Dragons died when they went between in a confused state of mind and never emerged. The fire-lizards of the Northern Continent were apparantly wiped out by the disease.
Two such infected fire-lizards (Lorana's Grenn and Garth) and one dragon (Lorana's Arith), in their confusion between, emerged at the start of the First Interval, near their mistress's ancestors, Tieran and Emorra, daughter of Wind Blossom.
D'gan, Weyrleader of Telgar, refused to allow sick dragons to stay in the Weyr, resulting in much higher fatalities fighting Thread, and, finally, the loss of virtually the entire Threadfighting force between when his own dragon lead the others into battle in a plague-induced confusion.