@Meddler: How would you make/balance a Curse effect?

I've asked you in the past about such an effect, but I'll refresh your memory on the subject.

Essentially, Curse would be a debuff that, when on a victim, causes it to take damage if it attacks or uses abilities. However, Curse damage cannot be fatal, and there'd be a clear visual indicator that essentially says "You're shooting yourself in the foot!"

But I have to ask how you would balance out such a debuff. Would you have be hard to apply? Would you have short duration? Would the Curse damage have to be low?

What are your thoughts?