Rescue discs are bootable discs which can be used to boot pc and remove any infections in case you end up with a non-bootable pc.
Some rescue discs also have automatic virus definition updates like bitdefender..
Its a good practice to have a rescue disc ready to eliminate the threats if your OS isn't booting even...
yes optimus prime some virus/torjon ,bot are difficult to remove but not impossible, theese root kits go deep inside and hide and you may not be able to remove theese virus ..etc from the os since the load with the os , so the sucess rate will be high when you try to remove the virus with such rescue media