When in hardmode (the latter half of Terraria's world progression), tiles affected by the corruption, crimson, or hallow will slowly spread their infection to nearby blocks. This means that ...

Defeating the Wall of Flesh activates Hardmode, a New Game Plus for Terraria. Hardmode differs ... Swarming is a major concern in the underground Hallow. Use blocks to temporarily close passages ...

which attack nearby players without discretion. Like the Corruption, the Hallow spreads rapidly unless contained by a four-block buffer of wood.