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 almost the entire world between them.

This process is part of the game's "tile update cycle": Every game tick (60 times a second), the game chooses a handful of tiles to update, which covers several processes including biome spread. The tiles above, and below, the surface level are selected in separate passes; As an approximation, "surface" tiles will average about one update every 140 seconds, while "underground" tiles average about one update every 830 seconds. Due to the random selection method, even the averages here are approximate, and the actual length of time between a given tile's updates can vary wildly. When an infected tile is updated, it is considered as a potential "source tile", which can infect multiple other tiles in a single update.

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 (see below for the latter case). 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 certain restrictions. Walls can also be converted by the initial Hardmode transition, or manually by the player. Unlike most blocks, walls can be converted even if they are already infected by a different biome.

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 ("grassy" and "sandy") for infection of walls, depending on the contents of the source tile. Both patterns are considered for each tile update, and can separately infect different walls. The behavior can also vary according to whether the tile is above or below surface level, and which infection is being considered.

Common Rules

 * In all cases, wall infection range is 2 tiles (a 5x5 rectangle centered on the source tile), compared to 3 tiles for most blocks.
 * Natural or placed Grass Walls (infected or otherwise), Jungle Walls, and Flower Walls, are all converted to infected Grass walls.
 * As with blocks, Hallow cannot infect Jungle walls. Pearlstone Walls also cannot infect other walls.
 * For a source tile to infect a wall, it must find a qualifying foreground block or plant with the infection in question (collectively, "biome blocks") either in the source tile, or within 20 tiles of it.
 * The qualifying biome blocks vary with context, but the following blocks always qualify: Grass, Ice, Stone, Sand, and Vines.
 * For Crimson, for sandy walls, or to infect any wall below surface level, the biome block must be in the source tile.
 * Otherwise (infecting a grassy wall above surface level with Corruption or Hallow), the biome block can be found anywhere in a 20 tile range (a 41x41 rectangle centered on the source tile).

Grassy Wall Infection
If the source tile includes any natural infected Grass Wall, natural infected Stone Wall (except Pearlstone Wall) or infected grass block, then it can attempt to infect a pure Grass, Flower, or Jungle Wall. Both natural and player-placed walls of these types can be infected.


 * Each of the infectious biomes is checked in turn: Crimson, then Corruption, then Hallow.  If the tile has a qualifying wall and a grass tile from different biomes, it can attempt to spread both biomes.
 * For each infection attempt, a single tile within range is chosen, and if it is a pure grassy wall, it will be infected.
 * Again, infection needs a "biome block" as above:
 * For Crimson or if below surface level, then the biome block must be in the source tile, otherwise within 20 tiles.
 * In addition to the list from above ( Grass, Ice, Stone, Sand, and Vines), Demonite Ore qualifies for Corruption. However, Crimtane Ore does not qualify for Crimson.
 * Most biome-specific plants (if infected) also qualify as biome blocks here. This includes Thorny Bushes, tall grass/flowers (that is, growing from proper grass blocks), Vile/Vicious mushrooms, and (again) Vines.
 * As above, Hallow cannot infect Jungle walls, and Pearlstone Walls cannot spread infection.

Sandy Wall Infection
If the source tile contains any Sandstone Wall, any Hardened Sand Wall, and/or any Sandstone Block, then it can attempt to infect a Hardened Sand or Sandstone wall.


 * Only hardened sand or sandstone walls, pure or with any infection, are converted this way.
 * Unlike with grassy walls, one biome is chosen, and then a single nearby tile is chosen for an infection attempt.
 * To attempt infection, the source tile must have either an infected wall and or an infected biome block.
 * Qualifying walls include Grass, Stone, Sandstone, or Hardened Sand. (Note that the first two imply a sandstone block from the first step)
 * Qualifying biome blocks include the list from above (Grass, Ice, Stone, Sand, and Vines), or Sandstone, but not other plants.
 * If the tile has different infections for wall and block/vine, Crimson takes priority, then Hallow. Corruption applies only if there is no competing infection.