Given what she's heard, the things she's seen, she thinks he's probably telling the truth and so she hits the button to let him in. Yesterday, someone came and stood in front of her door muttering to himself. If Mike were sick, she thinks he wouldn't be able to tell her to let him in.

Even so, she makes sure to keep her rifle close by and she makes sure to keep Clementine out of the room until she knows for sure. She waits for him near the door, opening it just enough for her to see into the hall.