|Researching the Cause|
Enter Malthus Books and Coffee Store in the Heritage District and go to the Manager's Office. Talk to Catrina, the manager. Find research documents, one item. The item is on the left side of the secret door which is located on the opposite corner of the entrance (see the map for details).
Unless you want to upgrade your fighting skills, it's pointless to try to kill all the zombies as they respawn very quickly. In fact, the more you kill, the quicker they respawn.