Thread

Thread was the name given to a voracious, non-sentient organism that inhabited the Oort Cloud of the Rukbat system. Its appearance was as thin, silvery threads that periodically rained down on the planet in an event that came to be known as Threadfall. Thread was a mycorrhizoidal spore that consumed any organic matter it could find, but was deterred by metal, plastic and rock, and killed by water, cold and fire.

Distribution

Thread's appearance directly coincided with the appearance of the Red Star in Pern's skies, a rogue planet with an extremely eccentric orbit that would bring it close to Pern in a cyclical nature.
Roughly every 250 years the Red Star would pass through the Oort Cloud located on the edge of the system. As it passed through the cloud, the planet would drag material with it, including the ovoids that would later become the organism known as Thread. About eight to ten years after passing out of the Oort cloud, the thread ovoids would be close enough to drop down to the planet. Intense heat and friction of entry into Pern's atmosphere burned off the outer casing of the ovoids, leaving attenuated filaments of the voracious life form to fall to the planet's surface.
As long as the Red Star's orbit took it past Pern, Thread would fall on the planet.
It takes Pern around 50 years to escape the Red Star's orbit as it passes them, lending the name of Passes, used to refer to the time periods when Thread was due to fall on the planet. The remaining 200-250 years of the Red Star's orbit, it was not close enough to Pern for the ovoids to fall, and came to be known as Intervals, the time period during which Thread was absent from the planet. Due to the Red Star's erratic orbit pattern, sometimes it would be taken out of the path of Pern for longer - around 400-450 years at a time - for Long Intervals.

Destruction

As mentioned prior, Thread was capable of consuming any organic matter it could find to come in contact with, which had devastating effects on Pern's ecology. The only substances it couldn't destroy were that of rock or metal, it could be drowned in water, or killed with extremes in temperatures, either by freezing it or flaming it.
(It also was deterred by plastic, but due to the chemical composition of Pern as a planet, plastic is in short supply and unable to be manufactured at present.)

Solution

Living on the open plains of the geographically unstable Southern Continent proved unwise to the colonists, who were eventually forced to relocate to the Northern Continent instead. The Northern Continent was less prone to tectonic activity, and the vast mountain ranges supplied a plethora of caves and warrens for humanity to hide in during Threadfall. Prior to their move, they also instructed AIVAS to look into all potential solutions for eliminating Thread completely before abandoning the continent altogether.
Additional solutions were also implemented to stop Thread from proliferating and destroying the planet's ecology as time passed:

• During Threadfalls, squads were often armed with flamethrowers to exterminate Thread while it was still in the sky.
• Any strands of Thread that made it past them would be destroyed by the Ground crews that cleaned out any burrows the Threads had made in the soil.
• As the power packs for flamethrowers dwindled, they were replaced by the dragons and their riders.
• Grubs were engineered and encouraged to proliferate throughout the Southern Continent, as a means to reinforce the natural ecology there and destroy any Threads that fell on the continent in humanity's absence.

The most important of these solutions was obviously that of the dragons and their bonded riders, as they would come to shape Pern's history and political scheme for thousands of years to come. Kitti Ping was the primary geneticist on the team that undertook the extraordinary feat, eventually creating the first clutch of dragon eggs before passing away shortly after.