At this point in the game, they should all have PTSD! While I'm sure Tanya saw some brutal things at the colony, I can't imagine her freak out to be purely fueled by those experiences. Especially knowing how hard refuge was to come by. After reading more of your posts, I'm totally believing her immunity to the infection. Perhaps her freak out was related more to the fact that the base would become aware of her immunity and in turn make her the guinea pig.

Then again, a person hiding their infection from others would be more careful about dropping certain information, like the fact that she was cold. I think they would go above and beyond to ensure everyone believed in their fitness.