Rust Plague

The Rust Plague was a mysterious disease that quickly and horrifically corroded any metal it came in contact with, including starships, buildings, medical facilities and Transformers. It was direct cause of Cybertron's loss of contact with its numerous colonies, putting an end to the Golden Age and ushering in the Age of Rust.

The Covenant of Primus
Alpha Trion wrote that, at first, the Rust Plague seemed almost trivial compared to warnings that Energon was running low due to supplying Cybertronian colonies and the worrying news that the Well of All Sparks was generating new life at a mere tenth of its former output.

Then came the news of a deep space transport that had been affected by a terrible disease that corroded any metal it came in touch with, including Transformers. Worse, this mysterious disease was incredibly virulent, spreading across entire worlds with terrifying swiftness. Alpha Trion called it the Rust Plague, as all that was left in its wake was rusting ruin. To the horror of the Transformers, who'd begun to think themselves a powerful race on the rise, the Space Bridges that connected their colonies helped the disease's spread.

Attempts to find a cure came to naught, as not only were any medical personnel killed quickly by the Plague, but medical facilities themselves rusted away and became useless. Cybertron was soon bombarded with cries for help and relief, angry claims that Cybertron had abandoned them and desperately hopeful assurances that a cure would be found.

The Council, led by Sentinel Prime even viewed a recording made by an investigator, who calmly walked them through an affected ship and pointing out the extent of the decay. He pointed them out to lumps of rust that he believed were once the crew, before presenting the corpse of his partner, who had been hale and hearty only a few hours ago when they'd boarded. Now, his partner's body rotted, his Spark chamber empty, his remains disintegrating at the slightest touch. The investigator then turned the camera on himself to show he had been infected as well and was doomed to die. He grimly advised the Council that any ship even suspected of being infected should be completely obliterated, and even then that might not be enough to stop the spread.

Sentinel Prime came to the heartbreaking conclusion that the only way to stop the Rust Plague before it hit Cybertron itself was to destroy the Space Bridges. Simply shutting them down wasn't an option, as it was not impossible that desperate souls might reactivate them in an attempt to flee, only to continue spreading the Plague. The Prime gave orders to all colonies and to all Space Bridge operators to self-destruct the Space Bridges. Any attempt to bring anyone through would be an act of treason punishable by death. Prowl was one of the operators, and fruitlessly tried to get his friend Bluestreak to return home before going through with his orders.

The plan worked, and after a long period of vigilance Alpha Trion concluded that the Rust Plague had been stopped from reaching Cybertron. However, the price was terrible. All contact with the colonies were lost, and the Cybertronian population was a mere fraction of that of the Golden Age. Worse, he observed that the experience had seemed to drain the life out of virtually all the surviving Cybertronians, noting that in Sentinel's case, it was as though the Rust Plague had affected his Spark rather than his body, robbing him of much of his former fire.