(Evil) biome spread

Hi all and sorry for noob questions.

I've been trying to wrap my head around biome spread mechanics for quite a while now and got few questions.

Do natural world generated backgrounds contribute towards biome spread and if so, on what level? As in if convertible block is o front of evil biome background block, will this cause convertion of the normal block?

Also, do dungeons contribute to biome spread or would I be able to use dungeon as one "layer of defence" again it?

Many thanks :)