The rust plague was a virus that attacked the Daleks' metal casings, destroying them.
It was created on Skaro when an accident at a weapons factory produced a radioactivecloud of metal-eating rust that destroyed a tracking station. The Daleks working there and a squad of Daleks on hoverbouts tried to destroy it. Their Brain Machine advised destroying the cloud by trapping it in a magnetic field and then spraying it with oil vapour.
This created a viral form of the cloud, which turned the Black Dalek Leader into a carrier of the plague. He unknowingly infected and destroyed all Daleks who came into contact with him. When the Dalek Emperor returned from Solturis, he realised that the Black Dalek was the carrier and had his casing reconstructed. (COMIC: Plague of Death)