The video game "DayZ" sold its one millionth download last week after being released on Dec. 16, 2013.
The version that is on the market now is the early-access alpha version, but DayZ's creator Dean Hall plans to improve it and release the beta version next year (video below).
According to Fast Company, DayZ is popular because it creates "psychological tension and emotional response" not previously experienced.
DayZ players only have one life, but must share their resources with (or steal from) other armed players and face deadly zombies. If a player is killed, he or she must re-download the game to play again.
The game is targeted for "survival-crazed sociopaths," notes ArsTechnica.com.
Hall says the idea for DayZ came from his own life-and-death experiences in the New Zealand army.
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"I kind of wondered, 'Why is it that as a soldier you have to go through and do all of this training?'" Hall told Fast Company. "Was there maybe a way to do some of this in a computer game?"
Hall tested the game on his fellow soldiers.
"I noticed how different they were behaving when their characters saved to a database, and they knew that the character was going to be back there every day," says Hall.
"They were suddenly arguing with each other," added Hall. "They were really tense. When someone got shot they were really concerned, they would try to help them. Normally in the simulation training we do, someone would die and they’d carry on with the mission. Well, if that happens in real life that’s not what happens, you know?"
Gamers have posted on Reddit how emotionally thrilling and realistic the game is, even comparing it to a "murder simulator" where regret, guilt and self-justification play a big part in the user's mind.