Nasal swabs reveal muted antiviral response in patients who developed severe COVID-19 Over the past 18 months, researchers have learned much about COVID-19 and its viral cause, SARS-CoV-2. They know how the virus enters the body, coming in through the nose and mouth and beginning its infection in the mucus layers of the nasal passageway. They know that infections that remain in the upper airway are likely to be mild or asymptomatic, while infections that progress down the airway to the lungs are much more severe and can lead to fatal diseases. And they have identified common risk factors for severe diseases, like age, gender, and obesity. But there are still many unanswered questions -- such as when, and where, the course of severe COVID-19 is determined. Does the pathway to severe disease begin only after the body has failed to control mild disease, or could it start much earlier than that?