The coronavirus is brand-new, so everyone is susceptible to it. Large numbers of people can get it — and spread it — without having symptoms, which makes it an especially wily foe. Public health officials have asked people to stay at home, not only for their own protection but to keep hospitals, and equipment such as ventilators, from being swamped. Pressure to loosen restrictions has mounted as case numbers in some states, including Pennsylvania and New Jersey, have fallen and unemployment has surged.