Everything, Everything (9780552574235) by Nicola Yoon

So allergic, in fact, that she has never left the house in all of her seventeen years. But when Olly moves in next door, and wants to talk to Maddy, tiny holes start to appear in the protective bubble her mother has built around her. Olly writes his IM address on a piece of paper, shows it at her window, and suddenly, a door opens.