The Thread is a super-parasitic disease created by the Delakoids in an effort to become immortal.
The exact time of the Thread's creation is unknown, but most scientists hold it to be around the year AD 66,666. On the planet Damos, Delakoids had been growing uneasy about their short(by galactic standards of course) lifespan of 500 years. Their scientists began work on an immortality-causing gene. To test it, they placed it in an infectious disease's spore under heavy guard.Success-it successfully mutated! However, an extreme side effect was noted-incurable paracital tendencies. Soon after, the spore escaped and ravaged the Delakoid race.
The infection cycle begins as a single spore combines with a simple life-form of/or organic structure. It releases dozens of spores, which drift off. This process may continue on for some time until a more complex organism, like a fly, is found. The spore then mutates into the Infector form. This seeks a new life form that is more complex, say, a Delakoid. It attacks the being, releasing thousands of supercells through two spiky tentacles into its body, basically disintegrating itself to create more of itself. These cells replace the old ones, reproduce, and eventually mutating its host. Most of the brain structure for memories is saved with any unusable info deleted and the tissue made into spore cells that gather into spore-creating organs on the back of the creature. All other organs are reconverted and the creature is officially a Warrior form. This entire process is completed in under 30 seconds.
Once a Warrior form grows too old to continue fighting, it develops into a Zombiefier. It is now a giant factory making millions of spore organs and Enslaver forms, which look like creepy, flying jellyfish with black eyes. In reality, this is a floating organ which latches on to its victim and make it into a Zombie form. This form goes out and infects other organisms.
Once enough forms have been created, they all start puking up massive amounts of spore cells, which form into Thread biomass, which proceeds to form into a Hive Mind-a massive, bloated glob of mouths, tentacles and biomass. Thread attacks suddenly become focused on leaders, brainiacs, technology and weapons. Biomass is assimilated into massive war machines along with technology, and intelligence is added to the Hive Mind's central brain.
Eventually, the Hive Mind begins drilling deep into the planet, hoping to find the Planet Seed. When it finds it, it begins to reproduce rapidly, so fast that in a flash of light, the entire planet is covered. It becomes a Hive Planet. All technology is used for its goals, and the Planet Seed is moved to the heart of Thread infestation.
A number of unusual Thread forms can be seen after this. They are the:
MEDUSAER. Thus is basically a Medusa-like creature that covers victims in hardened biomass.
GARGANTUA. This is just a giant mouth sunken into the biomass ground. Zombies come out of these.
SLENDERMAN. Wierd, huh? It acts as a harbinger of a Thread attack.
Once you're stabbed, bitten or touched, it's too late. There is no cure.
Currently, the only way to kill a successful Thread attack is by destroying the Planet Seed. All life on the surface will be extinguished, and the planet will start over. If you wish to survive, you must hide underground. You could also just repeatedly nuke the surface over and over, though most people aren't nuts enough to do that.
If fighting an in-progress attack, use flamethrowers, shotguns, tanks and plasma guns. Swords and other sharp objects are useful too.
Above all, the thread is the worst threat to the galaxy and should be stopped.