Vipers can be found in almost every outdoor environment on the island. They are a generic danger, having no distinguishing features or behaviors between them. No species has been officially identified and they could be native to the island.
The simplest way to identify a potential viper attack is the noise they make while moving through the grass, which sounds like a constant rustling of leaves. Looking around while moving backward is the easiest way to locate the animal, as they move slowly while approaching their target. They can almost always be avoided if their general location or direction can be ascertained.
Vipers do not continue to attack after striking. After a successful strike, the snake will attempt to flee, moving more quickly than on their approach. If left alone, they will reset after a few seconds and resume their aggressive behavior.
Vipers are the only source of venom drops, which are required for crafting the poison needle and lethal poison bait.
Vipers are unique in that they do not trigger a "Detected" warning on the HUD. They do trigger the "Chased" warning. This is dangerous because the player could still have aggro from another enemy, but has broken line of sight, when they aggro a viper. This could lead the player to believe that the "Chased" warning is generated by the original threat.