There's plenty that's still unknown about the lung-ravaging coronavirus COVID-19. According to Business Insider, one of the most vexing problems is whether people who have recovered from it can become reinfected. Experts say people who have been infected probably develop immunity. However, it's unclear whether it weakens over time. New blood tests can tell if people have had the virus by detecting antibodies. That's crucial for determining whether people self-isolating can return to work.