Dr. Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, called the striking down of the federal mask mandate on mass transit "a real challenge."

       "I think it is a real challenge because this is not going to be the last of the need for public health measures we've taken for any crisis," Osterholm told "This Week" Co-Anchor Martha Raddatz exclusively on Sunday. "And what could be the crisis of tomorrow?"

       The Transportation Security Administration announced last week that the travel mask requirement will no longer be enforced by the agency since the mandate was struck down. But the Department of Justice is appealing the decision after the CDC weighed in.

       "On precedence, this case is very, very important," Osterholm said. "I think the confusion around the mask mandate itself and how effective it is, is a secondary issue, but one that is still important, too."

       This is a developing news story. Please check back for updates.