Younger people are getting sick with Covid-19. States that had brief coronavirus lockdowns are struggling to encourage social distancing and mask-wearing. Many people appear to have embraced their usual summer rituals. Health officials are sounding alarms about a surge in cases racing not through nursing homes, but bars and house parties. Hospitals are filling with medically-vulnerable elderly—but also 20-somethings and patients in their 30s and 40s.

Some took the end of stay-home orders as permission to live their lives again, unimpeded. Phoenix entrepreneur Jimmy Flores, 30, spent the night of June 6 at a nightclub with friends, sharing drinks. Two days later, he felt sick. The next week, he was on a hospital oxygen tube after testing positive for Covid-19.

“I’m a young, active, healthy person with no previous conditions,” he said. “I didn’t take it seriously for myself. I was not practicing the social-distancing guidelines. I didn’t wear a mask. I thought I was invincible.” Mr. Flores said he went from not knowing anyone with Covid-19 to knowing 15 victims. After eight days’ hospitalization, he is recovering at home.