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.

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.

Coronavirus shutdowns have quashed nearly all other common viruses.

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.

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.

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.

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.

“We had the mildest flu season on record,” Glatt said.

The flu killed nearly 200 children last season.

Goza said that during an ordinary fall and winter, her practice usually orders box after box of tests for flu and RSV.

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.

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.

“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.

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?

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