Threadfall here are the wriggling, hungry offspring of the Red Star - a cosmic entity in and of itself, with morals and desires beyond human understanding. Threadfall whispers to the people of pern, drawing them near, turning them mad. It's only through bonding dragons that a person can shield their mind from the horrors of thread.
To a lesser degree, firelizards and whers will also protect a bonded human's mind.
Whether the dragons themselves are benevolent or parasitic is unknown, and no one bonding a dragon at Shigan is aware enough to question this. It is possible that a dragon's morality regarding bonding and fighting thread differs between individuals.
Sometimes a holdborn's mind is shielded slightly from the horrific truth of dragonkin, through sheer strength (or ignorance?) of will. The mind does not want to grasp the true appearance of dragonkin, and these special individuals can "look away", fooling themselves to the truth. These individuals are often the people that attract search riders.
During an interval, it's not uncommon for holdborn people to go generations without actually laying eyes on a dragon, which helps the population forget the horror.
Roughly 10 years ago, an unusually and untimely burst of thread* kicked the weyr into action. Many riders had become unpracticed during the interval and were lost to threadfall, further depleting Shigan's population. The current weyrleaders were killed during this fall.
Unprecedented and unrecorded, the thread stopped short of a true pass.*
It was enough to cause a flurry of action in Shigan. Flights were flown, new weyrleaders were chosen, search riders were sent out, and the weyr grew back to the minimum population required to fight thread. Only a few small wings, but discipline and training became paramount.
In the last few years, Pern has finally come into a true pass - and Shigan is ready to be a fighting weyr once again. With threadfall and the rise of dragons, the holdborn population of Pern are beginning to remember how monstrous dragons truly are.
*This "threadfall" was likely the result of a separated entity existing on pern itself, something that had grown from thread that reached Pern.
Dragons sometimes go feral and go between to join the brood permanently, taking their bonds with them.
Without a human bond, all dragons instinctively choose to join the brood. In this way, most dragons never truly die - and this is the real reason dragons go between if their rider passes before they do.