Bunny got infected and kept it hidden until you and her reached Metro. She was leaning against the wall as you were avoiding Zompiggy and getting the train back on. But...you saw it. One of Bunny's eyes was black with a white pupil...you lost Doggy. Your not losing her too.

Bunny: "Go. I'm infected...I don't know how long I have left before I turn but I can hold them off."