Packing pool parties and other Memorial Day events, many Americans marked the unofficial start of summer just like they did before coronavirus.
By Tuesday morning, more than 1,660,000 Americans have been infected with the coronavirus, and more than 98,000 have died, according to Johns Hopkins University.
In 17 states, the numbers of new cases are trending upward. Those states include Georgia, Arkansas, California and Alabama.
In 20 states, the numbers of new cases each day are generally going down. And 13 states, the numbers appear to be holding steady.
Some Americans took warnings from health officials very seriously -- wearing masks while in public, keeping their distance from strangers or celebrating the holiday weekend at home.
Those precautions are especially important because new research shows an estimated 40% of coronavirus transmissions happen before symptoms even appear.
And now that states have loosened or eliminated stay-at-home orders, 'it is up to every individual to protect themselves and their community,' said Dr. Stephen M. Hahn, commissioner of the US Food and Drug Administration.
'Social distancing, hand washing and wearing masks protect us all.'
'The disease can jump out at any time'
What used to be typical summer pastimes are now potential breeding grounds for coronavirus.
A cluster of new cases emerged after swim party in Arkansas. In Atlanta, several recent prep school graduates also tested positive for the coronavirus, including one who had friends over for a graduation party.
In Arkansas, one of the few states that never enacted stay-at-home orders, Gov. Asa Hutchinson said the state seemed to be experiencing a 'second peak.'
Despite the warning, crowds gathered over the weekend in Lake Hamilton, Arkansas, where Karen Lee said many people decided to forgo masks.
'We're all just embracing it,' Lee told CNN affiliate KARK. 'I could get killed by Covid today or I could get hit by a bus or a car tomorrow. I am practicing proper hand washing and hygiene.'
And officials from the World Health Organization say those who ignore measures such as social distancing are at risk of seeing a resurgence of the coronavirus.
'We cannot make assumptions that just because the disease is on the way down now that it's going to keep going down,' said Dr. Mike Ryan, executive director of WHO's Health Emergencies Programme.
'We need to be also cognizant of the fact that the disease can jump up at any time.'
A hallmark of coronaviruses 'is its ability to amplify in certain settings, its ability to cause transmission or super spreading events,' said Maria Van Kerkhove, the WHO's technical lead for Covid-19 response.
'And we are seeing in a number of situations in these closed settings (that) when the virus has an opportunity, it can transmit readily,' she said.
And while the coronavirus might transmit less efficiently in high temperatures and high humidity, that doesn't mean it will go away this summer.
Authorities are cracking down on rules
To avoid resurgences, state and local officials are trying to enforce restrictions in place across much of the country.
Health officials in St. Louis County, Missouri, issued a travel advisory Monday after videos surfaced from Lake of the Ozarks, where hundreds crowded a pool party.
'This reckless behavior endangers countless people and risks setting us back substantially from the progress we have made in slowing the spread of COVID-19,' County Executive Dr. Sam Page said.
Those who were in the area should self-quarantine for two weeks if they didn't practice social distancing, county officials said.
In Houston, authorities will enforce capacity limits after the city received hundreds of complaints over the weekend of bars and restaurants that were violating those limits.
In Rhode Island, police officers and park rangers were stationed at East Matunuck and Scarborough state beaches to ensure residents were wearing masks and following social distancing restrictions.
'It's definitely not the same beach experience,' Department of Environmental Management spokesperson Michael Healey told CNN affiliate WPRI.
There will no longer be lifeguards on duty, parking will be limited, and access to bathrooms, changing rooms and concessions is no longer allowed, WPRI reported.
2 months after its containment zone, New Rochelle reopens
New Rochelle -- once the epicenter of New York's coronavirus outbreak -- is starting to reopen Tuesday, Mayor Noam Bramson said.
The suburb north of New York City garnered national headlines after dozens of infections were traced back to one resident, an attorney who worked in Manhattan.
In March, officials announced a 1-mile 'containment zone' to help prevent the virus from spreading.
'The resumption of construction and retail pickup in Phase I will be a welcome boost to our local economy, and New Rochelle is already making plans to accelerate our recovery through both business and individual assistance,' Bramson said in a written statement.
'But it is premature to let down our guard. All of us must continue to act responsibly, practice physical distancing, use face coverings, and follow other guidelines intended to safeguard our health at both home and work.'
More than half of states investigate related illness in children
Meanwhile, officials in at least 26 states are investigating hundreds of cases of multisystem inflammatory syndrome in children, a condition doctors believe is linked to the coronavirus.
Those states include California, Georgia, Illinois, Louisiana, Maryland, Mississippi, Missouri, North Carolina, Texas and Washington.
A CNN survey of health departments across the country identified more than 350 confirmed and potential cases of the syndrome, almost half of them in New York.
Doctors have said children who may have the syndrome need immediate attention and will probably need to be hospitalized immediately.
Symptoms do not look like the classic symptoms of coronavirus. They can include stomach pain, vomiting, fever and possibly a rash.
The syndrome appears to develop two to six weeks after infection with coronavirus and affects mostly children who were perfectly healthy beforehand.