Valve, developer of Half-Life 2 & Left 4 Dead 2, released a patch for the Zombie Shooter Left 4 Dead 2. In the patch notes, it says “PZ Bots” enabled. What the hell are PZ?
My best guess would be Player Zombies, but what they actually mean is “Infected Players” / SI. The new bots use the aggressive survival mode behavior in versus and scavenge.
Aside from that Valve patched some issues with cheaters who would leave a server before a vote ended in order to avoid a server ban. They also fixed several maps and added new content to the SDK.
You can read the full patch notes here.