A few months ago, a young athletic guy came into my clinic where I’m an infectious disease physician and COVID-19 immunology researcher. He felt tired all the time, and, importantly to him, was having difficulty mountain biking. Three months earlier, he had tested positive for COVID-19. He is the kind of person you might expect to have a few days of mild symptoms before recovering fully. But when he walked into my clinic, he was still experiencing symptoms of COVID-19 and he could not mountain bike at the level he was able to before.