Activision, the video game publisher behind Call of Duty: Modern Warfare II, the popular first-person shooter, has taken the game’s servers offline due to a self-propagating worm infecting players using the PC platform.

Players began to notice unusual behavior quickly after the game was brought back due to popular demand, spurring conversation on the Steam discussion page about what appeared to be a self-spreading worm virus existing in the PC version of the game. Users began debating what the malware was — and whether it currently exists on VirusTotal’s repository — before Activision ultimately took the game down to assess the severity of the issue.

“Multiplayer for Call of Duty: Modern Warfare 2 (2009) on Steam was brought offline while we investigate reports of an issue,” stated a post on the X platform, formerly known as Twitter, from @CODUpdates.

Analysis of the malware confirmed that hackers are infecting the players of the game with a worm that spreads automatically within online lobbies. Researchers are investigating why hackers are distributing the malicious malware, but the problem has increased in severity, confirmed by a post on X by the company.

“In the last 24 hours, new detections directly targeting cheat developers at the source has led to over 14,000 account bans for cheating and hacking in Modern Warfare II and Warzone,” it stated on Friday.