The Infected
In the year 2020, the V-101 virus ravaged the world. In the year 2023, a variant more deadly than the original strain emerged. In response, a vaccine was created to fight the variant. The vaccine inadvertently lead to a new virus which alters the patient, transforming them into violent, cannibalistic mutants.
This new virus is passed via bodily fluid transmissions, mainly bites. Once bitten, it takes 3 days for the infected victim to transform. The infected feel no pain, except for the burn of the sun, which keeps them from coming out during daylight hours.
Very similar to pack animals, they hunt together, and seem to try to stay in the same locations when possible. They are known to hide in caves, sewers , under cars, and even densely packed forests.