Cosmic Rust (コスミックルスト, Kosumikku Rusuto) is a microorganism that devours Cybertronian metals. An infected Transformer starts growing a rust-like substance all over their body. Eventually, the corrosion causes the Transformer to waste away to nothing.
During the War, Cosmic Rust (technically called "Cyber-Oxidant") was created by Oil Slick to infect both Autobots and Decepticons alike. Unfortunately for him, when he tried it out at the Hydrax Plateau, he unintentionally infected himself. He was found by the medical officer Ratchet, who helped him create an antidote for the Cosmic Rust, unaware Oil Slick was the disease's creator. This ignorance was remedied when Oil Slick showed gratitude the Decepticon way—by trying to kill him slowly with an infection of Cosmic Rust. Ratchet survived and managed to steal a sample of the cure for his faction.