Amabel? (spoiler + analysis)

[thepaleking]

It could be that she was in fact a Tuurngait, but in order to further destroy your brain Clarence forces you to hallucinate Amabel's uninfected body.

It can be assumed that just about everyone in the facility was infected by the time they made that run for the surface, and Amabel's infection could have been similar to Philips in how it only gained control over smaller parts of the brain, rather than drive her completely wacko. While Philip was searching for the key card the infection could have simply overtaken her.