UPDATE: Infected is now available in public matches. It replaced Reinforce and you can find it on the main page of the Multiplayer menu.
How to play infected
Unfortunately, the only new Season 1 mode that's available in Public Matches right now is Enhance. Infected is not playable in public games. That doesn't mean it's not yet in the game, though. You'll just have to play it in a private lobby with friends.
To create a lobby, go to the Multiplayer menu in Modern Warfare. Next, create a custom lobby. Switch to the Alternate Modes tab and Infected should be in the list. From there, you can invite friends over and start the game. The game mode will hit public playlists in the future, although it's not exactly clear when. Other Season 1 content, like the new maps, Vacant and Shipment, and the new operator, Nikto, has yet to make an appearance in Modern Warfare, but is expected to be released before the end of the season.
What is infected?
Infected is a game mode that debuts in Call of Duty: Modern Warfare 3. It originally started as a private match game mode before becoming a full-fledged public game mode in 2012. Since its launch original appearance, it went on to appear in several other Call of Duty titles.
Like most incarnations of the game mode, Infected in Modern Warfare pits a team of survivors against a relentless onslaught of infected players. If a survivor is killed, he changes teams. Players on the infected team are only armed with a knife, but they have increased mobility to compensate for this. Survivors have access to guns, but most are close range weapons. Survivors win by staying alive until the end of the timer, and they lose if they are all killed.
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