Millions of Americans are getting ready to emerge from coronavirus lockdowns and venture outdoors to celebrate Memorial Day weekend at beaches, cookouts and family outings, raising concern among public health officials that large gatherings could cause outbreaks to come roaring back.

Medical experts warn that the virus won't take a holiday for the traditional start of summer. The Centers for Disease Control and Prevention continues to recommend that people stay home, avoid crowds and connect with family and friends by phone or video chat.

Dr. Seth Cohen, an infectious disease expert at the Uni...