When these things "take over", are they a) causing distress that just makes the host do things it wouldn't normally, b) taking control by producing hormones that alter behaviour, c) physically taking over the body by replacing nerves/muscles with mycelium and overriding anything the host's brain is trying to do, or d) taking over the brain of the host itself?

I'm not sure which option is the worst one. Each sounds just like a different level of "your body isn't yours anymore" hell.