The term adder refers to any of a number of viper species found around the world, of which the Pit vipers can be found in North America.
A number of adders lived in Raccoon Forest, particularly at the Arklay Laboratory where they could be found in trees in the courtyard as well as the underground tunnels. One of these adders was taken by the Umbrella research team and deliberately infected with the t-Virus for bio-weapons testing; the resulting giant became known as the "Yawn".
When the virus leaked out across the region, other adders were infected unintentionally.
Unlike the Yawn B.O.W., secondarily-infected adders experienced no major change in appearance. They did, however, experience an increase in the potency of their hemorragenic venom.[excerpt 1] Their increased aggression towards nonprey was coupled with the risk of t-Virus infection in snake bites. They also became abnormally fertile.
The snakes are first encountered in the courtyard in the original Resident Evil. As the player makes their way to the waterfall, they fall from the trees and attempt to poison them. Should the player be poisoned, there is a Blue Herb pot just down the elevator.
- ↑ Excerpt from Inside of BIO-HAZARD, p.43:
"ラクーン森林地帯に棲息するクサリヘビがクレイウィルスを保持した生物を獲物として、 ニ次感染したもの。外見的な変化はほとんど見られないが、 もともと保有していた出血性の毒が強力になっている。木の上や地下道に棲み、 獲物の体温に反応して姿を現わす。"
- ↑ Takeo (ed.), Inside of BIO-HAZARD, p.43
- ↑ Resident Evil: The Umbrella Chronicles (2007), file: "Adder Notes"