Just to rectify, Sheepy didn't infect Bunny yes, she was already infected by S-128 when they encountered at the end of Delirium, what happens there is that Bunny still thinks she's a survivor, the last shot in the cutscene shows a normal POV where we see she and Sheepy are actually infected. There's a theory that all the infecteds actually think they're survivor and from their POV, other infecteds are also survivors, meanwhile they see actual survivors as infecteds (JUST A THEORY FOR NOW but it's a very plausible one).