In the 14 October, 2008 update, a new flavor effect was introduced:
There are five levels of blood-staining, all with three possible variations: fresh, dried, or mixed. The more action a building has seen (and the less diligent its inhabitants have been about cleaning the blood), the higher the level of blood inside.
Blood changes from fresh to dried at the server reset. Mixes of dried and fresh blood occur when fresh blood overlays uncleaned bloodstains created on the previous day(s).
|1||The floor is flecked with fresh blood.||The floor is flecked with dried blood.||The floor is flecked with dried and fresh blood.|
|2||A few bloody handprints are smeared across the wall.||Splashes and prints of blood have dried on the walls.||Fresh splashes and prints of dried blood are smeared across the wall.|
|3||The floor and walls are marked with splashes of blood.||The floor and walls are stained with splashes of dried blood.||The floor and walls are splashed with old and new bloodstains.|
|4||The floor is slick with pools of blood.||There are dried, cracked pools of blood across the floor.||The floor is a slick, broken mess of dried and fresh blood.|
|5||Thick trails of blood are smeared across the walls and floor.||Thick trails of dried, smeared blood run across the walls and floor.||Thick trails of dried blood run across the walls and floor, smeared over by fresher splashes.|
It is also possible to see bloodstains when standing outside of a building. The only observed descriptions have been the following:
- The windows are splashed with fresh blood.
- The windows are splashed with dried blood.
- MISSING: windows mixed?
It is not yet clear what causes these bloodstains. Some believe the effect is tied to the amount of blood inside the building; others believe the effect is caused by injuring a survivor standing outside a building.
- Any successful attack upon a survivor, by either a zombie or another survivor, has a chance of causing bloodstains. One does not have to kill another character to create bloodstains.
- Creating bloodstains does not grant XP.
- There no special text acknowledging a "hit." Any effect must be inferred from changes in building's description.
- Not every successful attack will add to bloodstains. The exact percentage chance of causing bloodstains is unknown.
- It takes more than one successful, blood-spattering "hit" to increase the level of blood. The exact number of hits required for each level of increase is unknown.
- Blood changes from "fresh" to "dried" at the server reset.
- Fresh bloodstains can overlay dried bloodstains.
- If the "hit" is not enough to increase the amount of blood to the next blood level, the transition will be thus: dried level X --> mixed level X. (Further hits will result in "mixed level X" transitioning to "mixed level X+1")
- If the "hit" is enough to increase the amount of blood to the next blood level, the transition will be thus: dried level X --> mixed level X+1 
- Injuring or killing zombies does not appear to create bloodstains.
- It costs a player 1 AP to clean each level of blood. This action requires no special skills or equipment, and grants no XP.
- Cleaning is incremental. Upon cleaning one level of blood, the player encounters flavor text indicating the next level. Cleaning blood is thus unlike the "all-or-nothing" action involved in repairing decayed ruins.
- The flavor text received depends on the variety of blood present. Level of blood does not appear to impact the flavor text.
- Fresh blood: You wipe up the bloodstains.
- Dried blood: You scrape the dried bloodstains away.
- Any mix of fresh and dried blood: You clean up the bloodstains.
- Ruined buildings can be cleaned. (The ruin does not have to be repaired for cleaning to be sucessful).
- Bloodstains are not visible in dark, unlit buildings. If the building is dark, the ruins must be lit by a fueled portable generator before any cleaning actions can commence. If the player attempts to clean a dark building, they receive the follow flavor text: It's too dark to see if the building needs cleaning.
- Bloodstains of levels 1 - 2 may be cleaned at any time. In order to clean bloodstains of levels 3 - 5, any standing zombies must first be cleared from the building. If the player attempts to clean high-level bloodstains while zombies are present in the building, they receive the following flavor text: The zombie is standing in the middle of the blood stains.