The scourge invasion is a world-wide event. Zombies have invaded Azeroth, and don’t care about war mode or not, they are hungry for your brains.
The invasion is only a week long. Avoid it, join others to fight against it, or join the zombie horde.
Personally, I am on team Zombie. I see many more people killing zombies than I see people controlling the zombies. Don’t worry, the zombies have high health but we hit like wet noodles and have basically 2 attacks.