Gina and Rob Domaoal’s infant son was in day care for three months last year before the coronavirus shut it down. By the time Eli returned to a child-care center in early June, he had been all but quarantined for 15 months, so unaccustomed to being around other children that when they took him to a playground earlier this spring, he hung back at the edge and watched.

Eli had been back in day care for five days when the 20-month-old on Saturday suddenly had no interest in playing with his favorite trucks. He spiked a 102-degree fever. “He just wanted to be held,” Gina Domaoal said. “He felt like a baked potato that just came out of the oven.”

Tests showed Eli was infected with two viruses at once: a rhinovirus, which causes the common cold, and parainfluenza, another respiratory illness that can be more serious. Domaoal, who lives in Decatur, Ga., was not surprised. A friend had sent her young twins back to day care a week sooner. By that Friday, both were sick.

Such feverish children in the Atlanta area are part of a pattern of viral infections that pediatricians, infectious-disease specialists and epidemiologists have noticed cropping up this spring in the United States, especially in the South. These common viruses are showing up at a distinctly uncommon time of year — and sometimes with uncommon virulence among children whose immune systems did not begin building up familiarity with them while the pandemic kept people isolated at home.

The comeback of ordinary viruses is widely regarded as a dark underside of a season in which the coronavirus has been receding in much of the nation as vaccinations provide protection. As a result, people are shedding masks and abandoning social distancing — and resuming spreading viral droplets.

Last week, the Centers for Disease Control and Prevention issued an advisory warning that respiratory syncytial virus, known as RSV, has been detected with increasing frequency since March in 10 Southern states, plus Texas, Oklahoma and New Mexico. RSV is the most common cause of bronchial infections and pneumonia in children under age 1, with a season that typically runs roughly from November to early spring.

The CDC said in a statement Wednesday that it expects respiratory viruses to circulate more widely as people relax coronavirus avoidance strategies and schools and workplaces return to operating in person.

“It’s very unusual to see this volume of sick kids during the summer,” said Sally Goza, immediate past president of the American Academy of Pediatrics. “This is the time of year we normally are doing checkups for camp.”

Goza is part of First Georgia Physician Group, which has offices in Fayetteville and Peachtree City. Goza said she and her partners have been seeing babies and children with ear infections and strep throat. And as a member of the AAP board, she said that “it’s pretty much a consensus across the country that our volume of sick patients is higher” than normal for this time of year.

Maimonides Children’s Hospital in Brooklyn was among the first to notice a surge in RSV cases starting in March. Rabia Agha, a pediatric infectious-disease specialist, could not find much information on whether other U.S. hospitals were also having increases. But aware of similar surges documented in Australia and South Africa during their summer, she and a colleague began to gather data at their hospital.

The resulting paper, awaiting publication by the AAP’s journal, says that 295 children tested positive for RSV at Maimonides from March through early May, with a median age of 6 months. Strikingly, just over half of them required intensive care. Compared with the most recent pre-pandemic RSV season, the patients this spring have been more prone to need treatment in intensive care units and have tended to remain in the hospital about a day longer.

Parents can be surprised — and terrified. Charlie Hardin, host of an Amarillo, Tex., radio show, and his wife, Makayla, thought their 11-month-old was having spring allergies when he started coughing with a runny nose in late May. So did their pediatrician, who prescribed allergy medicine and suggested they give their baby boy, Moxxon, infant Tylenol.

Days later, with a fever joining his other symptoms, they took Moxxon to an urgent-care clinic, where an X-ray detected bronchial pneumonia. He was no better the next day, so they took him to the emergency room. Doctors tested him for RSV, said the positive result explained his pneumonia and an ear infection, and admitted him hours later when his fever rose to 102 degrees.

Moxxon was in the hospital for four days, fluids dripping into him through an IV. He was tethered to oxygen tubes as his oxygen levels dipped alarmingly low. Hardin said the days were especially hard on Makayla, who had gone through a risky pregnancy — prompting them to name the baby Moxxon after joking that their newborn had a lot of moxie.

He returned home Saturday, breathing better but with a cough still and little appetite. The Hardins postponed a weekend party they had planned for his first birthday Tuesday but gave him stuffed animals and a cupcake to smash.

Physicians and researchers say children, especially very young ones, are the patients showing up with viral illnesses, most often in doctors’ offices and sometimes hospitals, because common viruses can cause more-severe symptoms in them than in adults.

“The more seasons you have under your belt, the better your immune system is in fighting these things off,” said Stephanie Stovall, chief of quality and patient safety at Lee Health System, which has four acute-care hospitals and a children’s hospital in southwestern Florida. The system is seeing more than the usual number of respiratory cases among adults as well as children, Stovall said, but she suspects viral infections among adults may be undercounted.

“Adults with a mild cold almost never see a doctor,” said Aaron E. Glatt, chair of the department of medicine and chief of infectious diseases at Mount Sinai South Nassau, a Long Island hospital. “When a kid gets a cold with a fever of 104, the mom is quicker to seek medical care.”

The comeback follows a striking suppression of common illness during much of the pandemic, as workplaces, restaurants and other group settings shut down and many Americans followed public health advice to wear masks and keep a safe distance from others. “We had the mildest flu season on record,” Glatt said. “It’s one of the few benefits we’ve had from covid. The fact is the same measures we use to stop the spread of covid work extremely well to protect from other viruses.”

Goza said that during an ordinary fall and winter, her practice usually orders box after box of tests for flu and RSV. “We didn’t go through a box of tests,” she said.

Eli’s pediatrician in Decatur, Jennifer Shu, said she saw her first RSV case of the season on March 30. “It was the latest first case of RSV I’d ever seen” in 25 years of practice, she said.

The out-of-season illnesses are complicating insurance coverage, some physicians are noticing. At St. Joseph’s Children’s Hospital in Tampa, the RSV surge began last month, when 35 percent of the rapid tests for the virus came back positive — far above the 10 percent threshold the hospital uses to determine when RSV season has arrived.

John Prpich, a pediatric pulmonologist there, said that while there is no treatment for RSV, doctors can provide an antibody therapy called Synagis to prevent the virus from progressing to serious illness in patients such as premature babies and children with congenital heart disease. Prpich said insurers usually cover the treatment, which he said costs between $1,200 to $1,500 per month, during traditional RSV season, but some are resisting now.

Sometimes, he is finding, the best strategy is “siccing an upset and motivated mother on the insurance company.”

Though epidemiologists and infectious-disease experts had been expecting a viral surge as people loosened coronavirus defense strategies, “the real issue is, when we go back to our new normal, whatever that happens to be, is [the spread of viruses] going to all of a sudden explode or is it going to go back to the norm?” said Peter Katona, chair of the infection-control working group for UCLA.

Goza said she is concerned that, because people have not been interacting, there will be an uptick of slightly older children contracting illnesses usually concentrated among babies.

“Those patients who had not had a chance to catch these things will catch them at ages we don’t usually see,” she said.

Doctors are wondering, too, whether it makes sense to continue masks and other strategies many people adopted to protect against the coronavirus to slow the spread of other viruses. Glatt is a rabbi at Young Israel of North Woodmere, an Orthodox congregation on Long Island, and after his most recent weekly Sabbath service, he noticed more people wanted to shake his hand again.

“We as a society need to decide how much of an outlier it will be to be on a subway with a mask,” Glatt said. “The future will tell.”

For now, families are trying to cope with children’s illnesses for the first time in more than a year. The first time he started day care, Eli got sick after two weeks. He did not catch anything during the pandemic year, when the only child he saw was a 6-year-old in the other family in his parents’ bubble.

Domaoal discovered that caring for a sick toddler was different from caring for a sick infant. This time, “I thought he’d be more larva-like and accept that we were trying to make him better,” she said. Instead he was toddler-like defiant, rejecting Tylenol in grape and cherry flavors before she resorted to bubble-gum flavor.

At Shu’s office Monday afternoon, Domaoal hugged her toddler and held him down so the pediatrician could examine his mouth and ears.

Domaoal, an equity analyst, and her husband, Rob, a CDC scientist specializing in HIV, have been taking turns caring for Eli while he has been home sick. They said they think the day care has been careful, taking the 10 children’s temperature every morning and requiring parents to fill out a daily health questionnaire. Once Eli’s fever has been gone for a day, they will send him back.

“I don’t think it’s the day care,” she said. “I think it’s his immune system”