questions about poison

[Nappa]

How does poison work? If a unit gets poisoned does he take variable damage every turn until the poison is gone or is it a fixed amount of damage?

Is the save throw against the initial attack of poison or every turn the unit is poisoned?

Are multiple instances of poison possible from the same source? i.e. my Pan casts breath of the dragon against the same units twice.

If I cast foul vapors then breath of the dragon does the unit get poisoned twice?