Hidden room:

Once you have reached the end level 
(The Sewer), kill everything in the first 
area. Get to the next area with the zombie 
standing on the ledge. There is a boarded 
up door at this location. Punch the door 
and press B to enter a secret room that 
contains weapons, such as a dual hand gun. 
Note: A zombie is in the room, off to the 
right side. 


Hidden items:

Note the fan at the beginning of the 
Biological Weapon (Industrial Sewage) Plant 
level. Shoot it (you can not use a button) 
to collect some hidden items.