Medium Elemental: The first documented case of Covid-19 in the United States was reported half a year ago, days before early warnings from the U.S. Centers for Disease Control and Prevention (CDC) that a “very serious public health threat” loomed. Yet health officials had only a rough idea of how the novel coronavirus spread, who the disease affected most, and how to best combat transmission and provide treatment. Public messaging on the seriousness of the virus was at times conflicting and confusing, including the early advice not to wear masks. Six months later, scientists have a firm handle on how the virus spreads and what should be done to get the pandemic under control. Here are nine things we know about Covid-19 now that we didn’t know then.