Minecraft wiki entry for Silverfish, associated text: Silverfish spawners naturally generate in end portal rooms in strongholds. Silverfish can spawn from spawners only at light level 11 or lower, or on stone types they can enter (stone, cobblestone, stone bricks, etc.) at any light level, and cannot spawn within a 5 block distance of any player[verify]. This is the only renewable source of silverfish and infested blocks.