Note: This is pretty outdated, but the concepts remain the same

Since we were thrown into our world with only basic equipment, we have grown to conquer the surface, to endlessly defeat and torment the spirits that formerly ruled this land, and build massive creations in which people live their lives in. Do we want this plague to take over our hard earned land, do we want this flood to destroy everything we have worked for, do we want our buildings to be constantly harassed by creatures who hurt us and our colleagues ten-fold in force? The Answer is NO! We will fight for our land even before the battle has begun! We will fight them on the beeches! We will fight them on the Hills! We will never surrender!
But how do we fight for our land if the enemy we are facing grows stronger and stronger every second, never pausing the expansion they try and force. The answer: holding the line. If we defend what is ours from what they try and take, they will not be able to conquer anything.
Notes: Hallow is a disease just as much as the corruption in this guide
A worldviewer helps, but is not required.

Before hardmode, corruption is not a big deal at all. One can easily stop it by doing nothing, as it will run out of space to grow in ~30 minutes. Sunflowers are nice, but should anyone care to take the time to find one and plant it? The answer is simply, no. When the Dryad arrives she brings purification powder, this simple powder serves to remove corruption without eliminating the blocks. This can be exploited to mine shadow orbs before the nightmare pickaxe is obtained. A common misconception is that it makes the targeted blocks immune like a vaccine. This is false, it only restores blocks infected. Same goes with vile powder and the Hallow. Purification powder has upsides and downsides. While it does not require one to mine blocks, it can create a misconception that it normalized all blocks in reach, exploration is critical to confirm the restoration of all blocks to normal.
The danger of Corruption in this phase is minimal, but it can be catastrophic if one forgets to mine a single block of infected material, the corruption will spread like a plague in hard mode while you thought the area was clean.

The best way to prepare for the flood to enter your world is to not procrastinate. Mining with dangerous enemies makes the miner less productive and in constant danger. So the best time to dig hellavators and underground tunnels is before hardmode begins.
Corruption spreads across a three block gap, making wide tunnels necessary, two block wide access tunnels will not cut it, it is called a quarantine for a reason.
The first thing to think about is "where will the world suddenly become corrupted?" As soon as the wall of flesh is defeated, a large X or Y of corrupted/hallowed material forms in the earth, taking any blocks they can convert with them. Unlike surface corruption this shape will spread, through all levels of the earth. No place will be safe, eventually, the entire world except for naturally protected areas will be destroyed. Not only will the dryad be pissed, but everything will be different. No more blue sky, no more walks outside, no more bunnies. So the best way to know where to fight is to guess where it will form, be generous, as letting a small portion escape will create big problems.
Making tunnels is hard and sometimes boring work, but it is necessary for survival. So bring a friend, sit back and relax. Earn the highest tier armor you can obtain to reduce the time it takes to mine out everything.

While digging, it is important to know several tips that can seriously save time and protect your world better than just digging down for hours and hours on end.

• Try and keep thing lit, knowing where you are digging can help just as much as seeing the stray sand block that might act as a bridge for the hallow to get across and unearth the entire operation.
  
• Get rid of water, it could kill you, destroy light and slow down the mining process. Find a natural hole or cave where one could easily dispose of it. Make walls in lakes to prevent the water from following you down the shaft.
  
• Watch out for sand, it can kill you and act as a bridge for the infections. It might also take minutes just to clear several blocks, once you hit large sand dunes, dig under to save time.
  
• Be careful with lava, always drain it before trying to mine in the vicinity.
  
• Use the highest tier equipment possible. It will save tons of time and will help prevent tunnel boredom. You can use a nightmare pickaxe for dirt as it takes only one hit with it, but use the Molten pickaxe in most other situations.
  
• Bring a friend! It negates the boredom factor and can greatly speed up the operation.

Surface corruption will still exist when hard mode rolls through the door, so dealing with it is required. One does not need to eliminate it entirely as many have done, but it is critical that it is undermined so it cannot spread downwards into the earth. The surface corruption is very dangerous, a corruptor can follow you and fire its payload miles after you have exited, a complete dome is necessary to stop them before they exit. Also, Remember to cover the corrupt grass with clay to prevent corrupt vines from spreading, creating more corruption.
The most dangerous thing in the game reaps the best rewards. The demon altar generates the three hard mode ores underground, but in the process it corrupts or hallows a single tile in the world. This leads to huge corruption at unknown locations. It is easy to spot with a world viewer, but without it can be the most dangerous thing you had ever done in the game. Smashing six to nine is the ideal number, for only 1/3 of the material generates compared to the previous one. But after smashing, a systematic search is required to comb the exact location of the damage.

Well, you were not aware of the dangers that the new world has unleashed upon you. You have only mined a small amount of new ores before you begin to notice that the turquoise grass was not in that spot yesterday. Suddenly the grass below your feet changes as well. While pondering why this is happening in your study room, that distinct music begins to get louder and louder. It can’t wait any longer! Action must start now!
In the worst case scenario, you must begin digging as fast as possible, dig all around your house so it is like a floating island with a three to four block gap separating from the mainland. Try and get as far out as possible so you can’t hear music while indoors. Turn your island into a dome by surrounding it with two block thick glass walls. This will prevent corruptors from infecting home base.
If this fails, the only option is to move into the sky. But if you spot a harpy, you have gone far too high. Hope it does not come to this, but it is always an option.
Credits to Tieno for reminding me about this unwanted consequence

Well, you activated hard mode after all the time and effort you put into the defense. The job has begun, but it is never over. You will need to upkeep the defenses long after the world has changed.

• Watch out for corruptors, they will destroy the world and cause a mess without you knowing it. Kill them as soon as possible.
  
• Be sure to patrol the borders every so often, this will alert you to breaches.
  
• be sure to check the entire world for a demon altar problem. A world viewer helps with this, but is not required.
  
• Be sure to have fun in the world you entered in, conquered, and saved.

This is my world, it was my first world I made and I cherish it deeply. I have quarantined my world so no corruption can escape.

Made by: A cure for death

Do not reproduce any part of this guide without receiving consent.

Thank you for reading and happy mining!