Use commas to set off nonrestrictive modifiers. A nonrestrictive modifier is usually introduced by which and contains information that is not essential to establishing the meaning of what it modifies. See Restrictive and Nonrestrictive Modifiers.
Infectious diseases, which antibiotics and vaccines once promised to banish from our shores, have returned with a vengeance.
--Jonathan Tucker, "Invaders from the Rainforest: Review of The Hot Zone by Richard Preston," Technology Review