Biome spread

The evil biomes and the Hallow, unlike all other biomes, can spread themselves to other areas. They slowly convert (infect) certain susceptible tiles which are near their own, biome-exclusive tiles, to their respective biome counterparts, increasing the range of the biome. The exact rules for biome spread are subtly different for each of the three infectious biomes, as described at length below.

Collectively, these three biomes can be called the "infectious biomes". Blocks and walls which are vulnerable to infection are called "corruptible", those which cannot be infected under any circumstances are "non-corruptible". Blocks and walls which could be infected, but have not been, are called 'pure".

Biome spread starts immediately upon world generation, is greatly enhanced and sped up after defeating the Wall of Flesh (Hardmode), and is slowed down again after defeating Plantera. Once Hardmode begins, the world's evil biome and the Hallow have few restrictions on their spread, and without player effort, they will eventually take over the entire world between them.

Spreading tiles
Only the following blocks and plants of a biome are capable of converting susceptible tiles in their vicinity. Blocks and plants can convert blocks, plants, and walls, while walls can only convert other walls. None of the three biomes can convert each other's blocks, with one exception: Hallowed grass will always convert Crimson grass, but still stops at all other Crimson blocks.

Pre-Hardmode spread
In pre-Hardmode, the three biomes can only spread to a very limited number of tiles:

Hardmode spread
In Hardmode, the spread is greatly sped up and many more tiles become susceptible to conversion, as below.

Defeating Plantera for the first time reduces the speed of the biomes' spread to 1/2 (50%) of the default Hardmode speed.

Background Walls
Background walls cannot infect blocks, but they can be infected by blocks, and they can infect certain other walls within 2 tiles, if there are infected blocks nearby. Walls can also be converted by the initial Hardmode transition, or manually by the player. Unlike most blocks, any infected wall can be converted by walls or blocks from other infectious biomes.

In all cases, any wall undergoing biome conversion (including from Thrown Water or the Clentaminator) thereafter qualifies as a natural wall. This may affect enemy spawning, and perhaps the presence of the various Underground Desert biomes.

There are two overlapping patterns for biome conversion of walls, depending on the contents of the source tile. In some cases, the behavior can also vary according to whether the tile is above or below surface level.

Common Rules:

 * Natural or placed Grass Walls (infected or otherwise), Jungle Walls, or Flower Walls, are all converted to infected Grass walls.
 * As with blocks, Hallow cannot infect Jungle walls. Pearlstone Walls also cannot infect other walls
 * Corrupted and Hallowed walls above surface level require a foreground block or plant from the same infection within 20 tiles. Crimson walls, or any wall below surface level, require a foreground block or plant in the wall's tile. The qualifying blocks and plants vary with context, (see below), but the following always qualify: Grass, Ice, Stone, Sand, and Vines.
 * If the source (infecting) tile contains an infected wall and a different biome's infected block or plant, the target wall's new biome is converted according to the following priorities:
 * Crimson wall → Crimson block → Hallowed wall → Hallowed block → Corruption wall → Corruption block.

Regular rules:
If the Underground Desert behavior (see below) is not active, infecting walls requires the source tile to include any natural infected Grass Wall, natural infected Stone Wall (except Pearlstone Wall) or infected grass block. Anything else in the tile doesn't matter.


 * In this case, only the following walls are infected, but only if pure: Stone, Ice, Grass, Flower, Jungle.
 * Again, Hallow cannot infect Jungle walls.
 * If the source tile doesn't have an infected grass block, then another infected block (as above) or plant is also needed (as noted above) either in the source tile or within 20 tiles depending on height and the infecting biome.
 * Most biome-specific plants, (if infected) qualify for enabling infection. This includes Thorny Bushes, tall grass/flowers (that is, growing from proper grass blocks), Vile/Vicious mushrooms, and (again) Vines. This is in addition to the blocks listed under "common rules",
 * Demonite (but not Crimtane) ore blocks also count.

Underground Desert rules:
If the source tile contains any Sandstone Wall, any Hardened Sand Wall, or and/or any Sandstone Block, then the "Underground Desert" rules take effect. Again, anything else in the tile doesn't matter. In this case:


 * Only hardened sand or sandstone walls (pure or with any infection) are converted.
 * In addition to the "common rules" blocks, infected sandstone blocks permit biome spread, but most plants do not. Vines still enable spread, if they are somehow present.