They can't be reasoned with. They also cannot turn you into one of them unless you're exposed to the plague, which typically must be ingested. So don't eat any suspicious looking grains.
The typical ghouls are much faster than zombies you see in movies, and watch out for geists, who like to climb walls and attack from above.
I'm more concerned about the giants, abominations and frost wyrms, personally.
They're being led by a man called the Lich King. He's incredibly powerful and dangerous, so if you see a man in blue armor with a glowing blue sword, run.