The Possessed are the result of prolonged possession by demonic energy. When the hell portal opened, a wave of demonic energy went through the UAC Mars base, and any staff who couldn't make it to safety in time eventually got mutated into these creatures.

Side effects of demonic possession include:
Tearing of skin
Fusing of body parts such as the eyelids and mouth
Additional muscle tissue
Additional teeth
Formation of a large hole in between the eyes
Exposure of the gums
Decomposition of the body