Silverfish

Silverfish are small, bug-like hostile mobs which have silver skin and black eyes that hide in monster eggs found in strongholds and extreme hills. The blocks look identical to normal blocks, but when the block is mined, a silverfish pops out and attacks.

When attacked, silverfish may awaken other silverfish within a 21x11x21 area. This can lead to a terrifying swarm if there are enough in the vicinity, and an uncontrolled swarm can quickly surround and kill the player. Left alone, silverfish will re-enter in another block, transforming it into another monster egg.

Spawn

Silverfish don't spawn naturally, but spawn from monster eggs and monster spawners, which are only found in strongholds and randomly underground in extreme hills.

When spawning from monster spawners, they can only spawn in light level 11 or lower, except on top of stone, where they can spawn at any light level. They also cannot spawn within a 5 block distance of any player. These restrictions only apply when spawning from a monster spawner, and not from a Silverfish block.

Combat

The safest way to engage silverfish is by standing on top of a 2-block tall pillar, which will prevent the silverfish from reaching the player. The player then can then kill the provoked silverfish by using non-direct attacks, such as dumping a bucket of lava or dropping gravel. Non-direct attacks will not awaken other silverfish. Using splash potions of poison can be considered the worst way of combating silverfish, as each damage tick they take from the poison, it'll alert other silverfish in the area, immediately producing a large swarm.

If a silverfish is killed in one hit, it will not alert other silverfish, so a diamond sword is recommended to combat silverfish. Bear in mind that when silverfish re-enters a block, they are cured of any effects and restore any lost health.