The coronavirus pandemic taught Americans to wear good masks, avoid big crowds and test often to stop the spread of viral illness. Also crucial, but not fully understood early in the crisis, is that indoor air quality is key to reducing viral transmission. Fortunately, the Centers for Disease Control and Prevention released on Friday pathbreaking new guidance to improve ventilation in buildings, with specific targets that can make a difference the next time a novel virus strikes — or even when a bad flu is going around.