1. The first person to exhibit symptoms of an illness. The phrase originated during the AIDS crisis of the 1980s. My daughter was patient zero, the only one with a cold last week—now, she's very generously given it to the rest of the family.

2. By extension, the first case of or person demonstrating a particular issue or problem. This guy is patient zero for the crashing issue in the app, so let me see what he said in his original email.