Everything, Everything by Nicola Yoon

Madeline has a disease that keeps her in her house at all times: she's basically allergic to the world. She's homeschooled, she never goes out, and her mother and nurse have to go through a decontamination process before stepping entirely in the house or else Madeline will get sick. So when Olly moves in next door and starts talking to her through window messages and emails, things change and suddenly, Madeline's world is different. She doesn't know what's going to happen, but she knows that Olly's going to have her heart and it's going to be a mess.