This week marks 20 years since the September 11, 2001, terrorist attacks on the World Trade Center and the Pentagon. Researchers call this kind of shared disaster a “collective trauma.” Roxane Cohen Silver, PhD, of the University of California Irvine, who studies collective trauma and led a multi-year study on the mental and physical health effects of 9/11, discusses that research and how what we learned in the aftermath of 9/11 can inform our response to the COVID-19 pandemic, hurricanes, wildfires and the other large-scale disasters.