The Delta variant is more transmissible than the version of SARS-CoV-2 (the virus that causes COVID-19) that circulated for much of the previous 16 months or so, which means that it could spread faster in schools, just like it does anywhere else. Though it doesn’t seem to cause more severe illness (in either children or adults), Dr. Sean O’Leary, a professor of pediatric infectious diseases at the University of Colorado School of Medicine, says he’s concerned that kids could carry the virus back home to vulnerable family members, or in the other direction, putting teachers and staffers at risk. “I think it has the potential to be bad,” he says.