when players kill a zed, they earn in-game money and experience points. achieving certain types of kills, such as a head shot, may cause the game to enter "zed time," when all game actions for all players are slowed down for a few seconds, providing more time for players to adjust their decisions amid battle. once they spawn, zeds chase and attack players automatically. when damaged, players can restore their health using a medical syringe on themselves or having a teammate use theirs, among other regenerative items. once a player's health reaches zero, they will die and will not respawn until the end of the wave. the mission will fail if all players die before completing a wave. players gain monetary bonus for surviving a round, which they can use to buy and/or sell body armor, ammo, and weapons at a store, also known as the trader. the trader is only open for a limited time between waves and in certain locations around the map. the number of waves in a match can be configured, and four difficulty levels are available: normal, hard, suicidal, and hell on earth (ordered in increasing difficulty). a patch released in early 2016 included a dynamic difficulty option in which a computer-based "game controller" can alter the strength of subsequent waves, either making them easier or harder based on the players' performance.