Dragonsblood

Pern AL 50: the geneticist Wind Blossom is nearing the end of her long life and is painfully aware that the colonists are running out of the modern technology the settlers brought with them to Pern and that they are forgetting how to use what they do have. Society is beginning to revert to a feudal system and Wind Blossom is concerned that future generations may be hit by an illness they have no tools to fight and that mankind may subsequently be wiped off the face of Pern.
AL 507: Lorana is training to be an animal healer but when she arrives at Benden Weyr she impresses a golden dragon and becomes a dragonrider. However, a deadly and mysterious plague begins to wipe out the dragons, leaving mankind no defence against the deadly Thread which has just begun to fall. Lorana has two firelizards who manage to travel Back in time to Wind Blossom's era. Wind Blossom discovers what is wrong with the dragons and comes up with a cure. She also devises a way to leave clues for Lorana...