the racoon hospital tragedy

21.00[two hour's before the outbreak]
Dr George Hamilton was just done operating on a patient in the racoon city general hospital.
'I just don't understand what is happening, That's the third patient showing heartbeating after his death.
Maybe it's a sort of illness that is contagious?
If so we should look into that,But thatis gone be someone else his job now my work is over for today'.
George thought to himself as he changed from his his scrubs to his grey suit.
'Though day isnt't it?'Said dr Scott Jones while changing to his scrubs and puting on his stethoscoop and shoes.
'Yes your right about that,But my work for today is over,I think i am going to j's bar to have a well deserved drink.
'Have fun' jones said while they both leaved the locker room.
'Good night dr Hamilton'said nurse Kathy from behind the hall desk.
'Thanks and a good night to you to,Don't work to hard'said george.
'I won't'she said while george walked out the door.
23.00[the hour of the outbreak]
Hospital security guard's John walker was drinking coffee in the night security room, suddenly his partner keith miller called him to the over to the entrance.
john walked to the main hall and had a small talk with his partner about a strange behaving civilian just outside the hospital.
'He's making some weird sound'said Keith while pointing to the men outside.
'Ok i will check it'said john while grabbing his nightstick.
John walked over to the men'sir you i am gonna have to ask you to leave the premises'john said while trying to make eye contact with him.
Then the men stood right up but he was far away from being human now, his red eyes look at John.
'Sir are you all right?'John said while putting his hand on the men's shoulder.
Suddenly the men tried to bite John in the neck,John could just in time step back.
while the men tried it a second time Keith came and hit the men on the head with a flashlight.
And they both ran back inside,mean while the men along with five other people walked a bit crippled to the hospital making a scary sound and walking very slowly.
'We gotta lock the door to keep this thing out'said keith
John and Keith both locked the door.
after that Kathy came rushing to John from behind the desk.
'Are you alright'she said while looking at John's neck.
'I am alright,Just a bit shocked that's all'he said.
'Thank god'Kathy said while standing up, however she screamed when she saw the six people wrom outside knocking on the door.
'Don't worry,That's double glass they won't break it from there'Keith said while looking at the glass.
'But...' Kathy said when she was interupted from a loud scream coming from the first floor.

what could that scream be?