Covid-19 acts in a way that scientists are still trying to figure out. In some people who are infected, symptoms are mild -- like a common cold. Some are completely fine. In others, the infection can be fatal, stopping the lungs from functioning and causing the body to shut down. So why are symptoms so mild in some people and deadly in others? It turns out there's a tipping point -- a moment where the virus moves from one part of the body to another -- that takes the infection from manageable to fatal.