Thread is brought to Pern by another planet in its solar system, the inexactly named Red Star. The Red Star has a two hundred and fifty Turn (Pernese year) elliptic orbit around its sun, Rukbat. When at aphelion, the Red Star travels as far as Rukbat's Oort Cloud, the natural habitat of Thread. As the Red Star moves through the Cloud, some of the Thread falls into the planet's gravity well, remaining there as the Red Star leaves the Cloud on its journey sunward. As the Red Star approaches perihelion, gravitational perturbations and outgassing from the Red Star's interior launch Thread on trajectories that impact Pern.
Threadfalls occur on Pern for about fifty Turns while the Red Star is near Pern's orbit. This is called a Pass, while the two hundred Thread-free Turns during which the Red Star is farther away are called an Interval. Twice, the Red Star did not come close enough, leading to a Long Interval of four hundred and fifty Turns. McCaffrey's first Pern novel, Dragonflight, begins near the end of such a Long Interval. Long Intervals are of such duration that, by their end, most of the Pernese population believes Threadfall has permanently ceased.
Two extra Long Intervals were caused when dragons were sent back in time by AIVAS to detonate large explosions on the Red Star's surface. The explosions perurbed its orbit, causing the long intervals. A third explosion in the present completed the calculated change in the Red Star's orbit, ensuring that thread never again threatens Pern once the present pass is complete.
Thread remains dormant in inner-system space as a small, icy ovoid, but is reactivated by passage through the atmosphere. Heat and atmospheric friction burn off the outer shell, releasing "thread-like" strings which float down to the surface in sheets, tangles and clumps. The size of a single strand is initially comparable to a long, silver strand of yarn. However, while feeding, Thread grows visibly, being described as a "heavy hawser" rope or "wriggling sausages" about 10 cm by 3 meters in size.
Structurally, Thread is composed of many thin, tightly wound filaments within a very fine outer shell or film. Complex proteins (presumably including proteases, nucleases, glycolases and adipases linked to molecular motors and fibrils) allow Thread to consume any carbon-based substance through direct physical contact as well as providing wriggling mobility. Contact with Thread results in "Threadscore," similar to a chemical burn.
A small clump of Thread can devour a fully-grown cow in moments. Feeding Thread grows rapidly, pulsing with sickly grey and green colors as it consumes. The exterior shell thickens with growth, and Thread dies from the inside out, suggesting that Thread's explosive consumption interferes with its metabolism. The shell thickens and hardens in death, while the inside undergoes an "unravelling/melting" process that leaves a foul stench. Water drowns Thread quickly, leaving a "soggy, bubbling mess".
Thread has no brain and is not sentient. The majority of Thread dies after landing, due to starvation. However, if Thread survives to begin its feeding cycle on the organic component of soil, the result is a burrow that multiplies extremely rapidly. A single burrow can devastate multiple square miles of land before perishing.