User blog comment:Ritzman/A Question That Confuzes me and probaly you maybe./@comment-4674838-20120117024321

Even if you clear all of the corruption in the world, I'm fairly certain that brand new corruption will be generated when the Wall of Flesh is defeated. (I haven't tested this myself, so I'm not 100% sure, but it's written on the Terraria Online wiki here. Terraria Online generally has more accurate/up-to-date info than this wiki, so if you need to know something very particular, or if you can't find it here, you should always check there.)

Also, when you're in the Underworld, don't shoot at Voodoo Demons when they're flying over lava. Wait for them to fly over land before you kill them, that way you know the Guide Voodoo Demon won't fall into lava. (Keep in mind that you can build bridges over lava)