Zombies are scientist?!?!

[Peanuckle]

When you make contact with her at her computer, she repeats herself about the symptoms of infection, which is itself a symptom of infection. This and subtle other clues in her conversations implies that she is already infected.

Thus it is entirely possible that she was resisting, but lost the battle when you got to her. Since it'd be fairly early in her infection, the transformation would not have taken place.