Tess gets shot and she was infected shortly before. She is super dead. Only thing she didn't get is a professional autopsy - you can actually go back and check out her body yourself, but I guess Joel isn't a doctor.
Assuming this ending was planned, Tess probably wouldn't of had such a clear-cut death. You'd hear gunshots, assume she died, and like Marlene she would turn up later in the game. The narrative would have been quite a bit different, because either Tess would track Joel, or Joel would return to the original quarantine zone.