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 was scheduled to be infected with the t-Virus for bio-weapons testing at some point, however this adder escaped and evaded recapture. During the t-Virus leak, it was infected and the resulting it to become a giant monster known as the "Yawn", using the Mansion as a hunting ground.
When the virus leaked out across the region, other adders were infected unintentionally.