In an average year, according to the Centers for Disease Control and Prevention, about 21 million Americans get the norovirus, with classic stomach flu symptoms such as vomiting and diarrhea. Eight hundred die. Symptoms come on very suddenly, within hours after a person has been exposed to it. Because no one has immunity to this new strain, more Americans — perhaps 50 percent more, the CDC says — could become violently ill. While the flu is spread mostly in the air by sneezes and coughs and a person needs to breathe in as many as 1,000 virus particles to get...