The Left 4 Dead series has sold more than 12 million copies.
Valve writer Chet Faliszek revealed the sales figure in an interview with VG24/7.
"We just let the game speak for itself because the Internet has made this thing where you can’t pull the wool over people’s eyes any more," said Faliszek. "They’re going to know you’re hyping. But when a game does really well at prerelease, they're going to know that you’re not just talking a bunch of PR crap.
"That’s amazingly cool," he said, "and that wasn’t because I was out there like, ‘Oh my god! This game is amazing! Buy it!’"
The series consists of two games and the sequel's various add-ons, including "The Passing" and "The Sacrifice."
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