In the cut-scene introduction to Ballistic Trauma, we see Papa Caesar on his mobile phone telling someone that Detective Washington and his associate (Agent G) are on their way to the hospital. Papa Caesar explains to the person on the phone that he has a plan to eliminate any evidence of their evil plot. Here we see the Screamer suspended in a large tube of liquid.
Throughout the level, the Screamer can be seen stalking the players throughout the hospital, randomly popping up in various windows and peering through certain doorways. During these 'cameos', the Screamer cannot be harmed, nor can she harm the players.
At the end of Ballistic Trauma, the Screamer is faced by our heroes in the bowels of the hospital in an empty, partially flooded operating theatre.
In the 'Director's Cut' version of the level, Detective Washington and Agent G encounter the Screamer prior to the actual fight. She is crouched in the corner of a dark corridor in the hospital, and when they approach her, she stands up and displays her not-so-lovely singing voice - smashing windows and sending our heroes across the corridor into unconsciousness.
As seen in the cutscene before the level, she is incubated in a large test tube. She looks to be a female patient with her disoriented probably due to the tests she was worked on. Her vertebrae can be seen on her back making her posture leaning. The mutation from the formula greatly enhances her vocal cords granting her the powers to scream, hence her name, in a very loud tone. However, her body isn't tough enough to deflect gun ammunition so she use the method of misdirection and false imagery to distract her opponents when being threatened.
The screamer, a boss, a shouting instrument, summons the doctor zombies and all. She is the boss in Ballistic Trauma. Her weak point is shooting her whole body. On the first attack, she will appear then coming near to you. The best way to use in this attack is an SMG. On the second attack, she will appear and will also summon dr. zombies. The best way to do is shoot the blocking zombies by an SMG then when there are no zombies, use a shotgun or an auto shotgun that's upgraded to shoot her before she screams to you.
In the extended cut, the boss fight has a slight increase in difficulty, as one of her attack patterns is changed from changing between nurses and having to shoot her, to charging at players through several flashes accompanied by false images. With little time to react, one must choose which projection to shoot, otherwise they will be maimed in a horror movie like fashion.