Minecraft wiki entry for Infested_Block, associated text: When mined with a tool not enchanted with Silk Touch, an infested block drops nothing and spawns a silverfish that immediately attacks the player. If the player attacks a silverfish directly with a sword, bow, or potion of Harming, or when a silverfish takes Poison damage, nearby infested blocks break, spawning more aggressive silverfish. If the silverfish is killed in one hit, then nearby infested blocks do not break, and no silverfish spawn from them.