Snakebite is a gear perk that raises your weapon damage against poisoned enemies. It is a key piece of a poison build, letting you finish targets quickly once the gas or poison rounds have hit them.
Your shots hit harder against anyone who is poisoned. Poison first, then shoot, and fights end faster.
Wear the Hazmat Gloves MK II in the hands slot. Gear perks are fixed to the piece of clothing that carries them, so you get this one by wearing that piece, not by unlocking it in a menu.
Relapse heals you when you kill poisoned enemies, and Toxic Gadget Pocket lets you carry more poison throwables.