Infection generally starts in the nose. Once inside the body, the coronavirus invades the epithelial cells that line and protect the respiratory tract, said Taubenberger, who heads the viral pathogenesis and evolution section of the National Institute of Allergy and Infectious Diseases in Bethesda, Maryland. If it’s contained in the upper airway, it usually results in a less severe disease.

But if the virus treks down the windpipe to the peripheral branches of the respiratory tree and lung tissue, it can trigger a more severe phase of the disease. That’s due to the pneumonia-causing damage inflicted directly by the virus plus secondary damage caused by the body’s immune response to the infection.

“Your body is immediately trying to repair the damage in the lung as soon as it’s happening,” Taubenberger said. Various white blood cells that consume pathogens and help heal damaged tissue act as first-responders. “Normally, if this goes well, you can clear up your infection in just a few days.”