Upon which, monsters become harder, blood moon/invasion becomes tougher, new monsters (like friggin wyverns) start to spawn, and the Hallowed and the Corruption begin to fight for domination of the land, destroying all other biomes in their path. There are a few types of blocks that are immune to corruption/hallow transformation, and the only effective way to save the world is to (1) decide which side you want to win, or (2) isolate both infections from spreading to other biomes by making a 3-4 gap/block barrier of infection immune blocks.
It's pretty sad to see the land you know and love be robbed of its personality. I made the decision to let the Hallow/Corruption fight it out in my first world, but it was hard to just let that happen knowing that some of my favorite places would forever be changed. That's why I decided I would create my own mini worlds in the sky. Hence my bias toward making our home base there too.
NPC's in the sky home reduces wyvern spawns in the area by a large degree. Fighting against them isn't too bad when they're spaced out.