the scientists have been working and researching about the plaga creatures and its effect on humans,but one day the scientists diden't relelise that the plaga have magore effects on humans,so they enjected the plaga into a dead body then the nightmare started,the plaga has infected the body and used them to kill and infect other bodies,so you will have to go to the plaga labs and see whats really happening and stop it before the plaga creatures spreed the infection.