When will life return to normal, or at least a new normal? A major answer to the question of when — and how — Americans can return to public places like work and school could depend on something called an antibody test, a blood test that determines whether someone has ever been infected with the coronavirus. People who are believed to be immune may be able to safely return to work. It would be especially important to know which health care workers are protected from getting infected and could continue to care for sick people. The Centers for Disease Control...