The Axis was a hub of a spindle of dead-end timelines where history has spiralled off due to catastrophic directions being made. This results in them being cut off from the rest of reality.
Typically, such changes occur due to the result of meddling by the Time Lords or by the immortal, omnipotent Overseer of the Axis who has little love of their kind. Inside the Axis resides the Oracle room where a set of locked doors are present and represent the dead end timlines being locked away. It became infected when a mad inhabitant of one of the timelines entered the Axis itself. (BFA: The Axis of Insanity)