She joins the project because of Lucas. She's not supposed to know about it, but after the run-in with the tomatoes, she's suspicious. After the wedding, when he's called away from their home too often, Rachel starts looking for answers.
She takes Lucas' brush, pulls out hair samples, and, after analyzing the DNA, sits down and takes a lot of deep breaths, counting to ten three times over. It helps.
Molly's welcoming; she has no other choice. The infection has spread, Dr. Fenway is overworked, and they need a doctor who's got a personal investment in stopping this.