During a phase of video game making, a developer at some point had to draw some concept art to understand what they would be getting themselves into. In most cases, multiple sketches are drawn of an in-game model with only a few of them actually being used in the game. It appears that in those early development stages, Naughty Dog had some different ideas for the infected, a few that could have possibly changed the game completely from what it is now.
Down below you’ll see some concept art that are unlocked in the game. Can’t say that these are spoilers considering that we have seen what the infected look like, but maybe perhaps they’ll use these in the sequels (if there’s going to be one).