After 5 years with COVID-19, health experts say there is still much to learn about the virus that causes the disease. At the height of the pandemic, hospitals were overwhelmed with patients with severe cases of the respiratory virus including life-threatening complications. It was believed that a certain type of lung cell made some people more susceptible to severe infection but now that is no longer the case. KTVU’s Heather Holmes speaks with Stanford scientists Catherine Blish and Mark Krasnow about their critical new research that found a different suspected source of vulnerability.