Isolation

The plague destroyed the world you once knew. This unknown bacteria destroyed the lives of many people who lived in various countries around the world. When the plague was first identified by doctors, researchers started working on a cure immediately. The infected people were transported into hospitals to isolate them from other people. But this was not enough to stop it. Researchers failed in finding a cure, and there was nothing anyone could do. Slowly the plague started affecting all countries worldwide, and then that's when civilisation collapsed, and that is when the plague infected your country.