Achievements

Left 4 Dead includes Steam's achievement system, recording players' accomplishments and mishaps. Survivor players receive merits for accomplishments such as completing a scenario without using any health packs or helping a fellow injured Survivor. Demerits are given for mishaps such as shooting a teammate or walking into friendly fire.

The Infected receive merits for accomplishments such as killing or wounding a survivor, combined attacks on Survivors, or causing a swarm of AI-controlled Infected to kill a survivor. Demerits are received for mishaps such as harming a teammate, avoiding the survivors for a certain period of time, or purposely helping the survivors.

List of achievements
Here's a list of current achievements. Valve is probably going to add more achievements later.