Guide:Maintaining world purity

Prior to the activation of Hardmode, which is activated by defeating the Wall of Flesh, there lie only a few, relatively small areas of Corruption (or Crimson) on a world. However, once Hardmode is activated, two large strips of both Hallow and Corruption/Crimson appear cutting through the entire world diagonally, reaching from the Underworld to even Sky Islands in its path. Not only that, but the Hallow and Corruption/Crimson generated (as well as existing areas) will spread rapidly through any grass, sand, and stone nearby.

Dealing with the strips that generate upon defeating the Wall of Flesh would be difficult and time consuming on its own, and smashing a Demon/Crimson altar with the Pwnhammer (or more powerful tools later on in the game), has chance to make a single, random block of your world turn Hallowed or Corrupt/Crimson, which given enough time, will spread to form small new areas to deal with.

On a regular playthrough, It's not important to focus on this too much, as it takes an extremely long time for the two biomes to take over a significant part of even Small worlds. However, since it can potentially ruin worlds that have been played on for a lot of hours, some players focus on eliminating this problem.

Corruption/Crimson
In Hardmode, the Corruption/Crimson spreads far more rapidly; it will corrupt everything in its path except for the Hallow and non-corruptible blocks, such as Wood and Bricks. It can also turn Mud Blocks into Dirt Blocks, which means that the Jungle can also be affected by the spread, however it takes a lot longer than with any other block.

The spread of the Corruption and Crimson slows down to 33% of its original speed after killing Plantera, significantly reducing the threat.

The Hallow
Despite its friendly looks, the Hallow is only slightly less dangerous than the Corruption or Crimson. It spreads almost completely in the same way as the Corruption except that it cannot spread through the Jungle. Because of this, many players consider it the "light" Corruption. However, if you do not want to take action for keeping your base pure, Hallow is a better choice than Corruption/Crimson, so you may want to plant some Hallowed Seeds. The Hallow does not uproot or prevent native forest plants from growing, nor does it prevent NPCs from moving in as opposed by the Corruption/Crimson.

The spread of the Hallow slows down to 33% of its original speed after killing Plantera.

Pre-Hardmode
As soon as Hardmode begins, The Hallow will appear, and both it and the Evil biome will spread rapidly. So, before you switch to Hardmode, it is easier for you to contain the it first. The simplest step, and often the first, is to dig a tunnel 6 blocks wide completely surrounding the generated Corruption/Crimson areas, and then make a "barrier" with blocks that aren't affected, such as Wood or Stone Slabs. It is also a good idea to build these tunnels in areas of interest, such as the Jungle or the player's Base, since the biomes will easily spread to them.

Hardmode
As soon as the Wall of Flesh is defeated, two diagonal strips of Hallow and Corruption/Crimson will generate in a V shape. The location of these strips and distance between them is completely random; but if the player has done a lot of previous underground exploring, the affected areas will be very clear. These diagonal strips will begin spreading as soon as they appear, so they can create a mess if left unattended. However trying to contain the spread at this point is a bad idea, as the new enemies can easily kill players at this point, and the tools they could be using are very slow for the task. So it's much better to wait until much better gear is acquired.

Smashing a lot of Demon Altars/Crimson Altars will make it a lot easier for you to find the new ores quickly, minimizing the spread before Plantera is defeated. However, breaking an Altar has a chance to turn a random block into its Hallow or Corruption/Crimson counterpart. This will barely speed up the overall spread, however the block can spawn anywhere in the world, potentially affecting an area the player has tried to keep safe.

If the Hallow didn't spawn in the side of the jungle, it isn't a bad idea to artificially create one around it to prevent the evil biome from getting there first, as mud isn't affected by its spread.

Once a Mechanical Boss has been defeated, the Steampunker will arrive, which will give the player access to the Clentaminator, which helps when "cleaning" massive parts that have already been taken over. Note that this tool and its Solutions are very expensive, so the player should have a big amount of money before starting.

Complete Conversion
If you want to make cleansing your entire world a feasible task, the Clentaminator is a must.

The most foolproof way to completely convert or purify your world is to make multiple Hellevators every 116 blocks(the maximum range of the Clentaminator), then glide down each one using Wings or Feather Falling, while spraying both sides. The Clentaminator also produces a decent amount of light that can completely go through blocks, this allows you to completely map out your world, as well as completely purify it.

As good as completely eliminating the spread might sound, leaving a part of the world with Corruption/Crimson and Hallow isn't a bad idea if contained properly, since there are a lot of items exclusive to these biomes and the player may need those later.