Several people who attended a huge sandbar party on Michigan’s Torch Lake for the Fourth of July have since tested positive for COVID-19, health officials say. But the event was so crowded—with thousands estimated to have attended, and most of them in close proximity—that health officials say it’s impossible to track all the people the infected attendees came in contact with. Video from the festivities went viral over the holiday weekend, with many shocked to see so many revelers packed so tightly together as they danced without face masks. “This situation reminds us of how important it is to take precautions such as avoiding large gatherings whenever possible especially without social distancing and masking,” said Lisa Peacock from the Health Department of Northwest Michigan. The department is now calling on those who attended the event to monitor themselves for symptoms and get tested if they were in close contact with others or didn’t wear a face mask.