The third-floor off-campus apartment where I'd failed to live had no buzzer, making it hard for my friend Laura to let her visitors in. That’s what I was, our junior year in college: a visitor. Not the roommate she’d asked me to be, not the one she called out to in the middle of the night when thick, sticky mucus caused by her cystic fibrosis--a genetic disorder she’d lived with her entire life--clogged her lungs…