Time sure does fly by when you're having fun (also guts). Killing Floor is celebrating its fifth birthday, and Tripwire has shared an infographic of impressive numbers and trivia to mark the occasion. Did you know that Killing Floor was originally released as an Unreal Tournament 2004 total conversion mod in 2005? Or that the retail version we know and love, and which went on to gain 3 million players, was first developed by a team of 10 people in just 3 months?

They grow up so fast. Five years and more than 20 billion dead zeds later, Tripwire Interactive is now at work on Killing Floor 2. For more on that game, make sure you catch up with Wes' exclusive first look , and his chat with Tripwire about how it aims to make gaming's most realistic guns .

For more Killing Floor numbers, scroll down: