By Frances Stead Sellers

At one point last month, children were admitted to Yale New Haven Children’s Hospital with a startling range of seven respiratory viruses. They had adenovirus and rhinovirus, respiratory syncytial virus and human metapneumovirus, influenza and parainfluenza, as well as the coronavirus — which many specialists say is to blame for the unusual surges.

“That’s not typical for any time of year and certainly not typical in May and June,” said Thomas Murray, an infection-control expert and associate professor of pediatrics at Yale. Some children admitted to the hospital were co-infected with two viruses and a few with three, he said.

More than two years into the coronavirus pandemic, familiar viruses are acting in unfamiliar ways. Respiratory syncytial virus, known as RSV, typically limits its suffocating assaults to the winter months.

Rhinovirus, cause of the common cold, rarely sends people to the hospital.

And the flu, which seemed to be making a comeback in December after being a no-show the year before, disappeared again in January once the omicron variant of the coronavirus took hold. Now flu is back, but without one common lineage known as Yamagata, which hasn’t been spotted since early 2020. It could have gone extinct or may be lying in wait to attack our unsuspecting immune systems, researchers said.

The upheaval is being felt in hospitals and labs. Doctors are rethinking routines, including keeping preventive shots on hand into the spring and even summer. Researchers have a rare opportunity to figure out whether behavioral changes like stay-at-home orders, masking and social distancing are responsible for the viral shifts, and what evolutionary advantage SARS CoV-2 may be exercising over its microscopic rivals.

“It’s a massive natural experiment,” said Michael Mina, an epidemiologist and chief science officer at the digital health platform eMed. Mina said the shift in seasonality is explained largely by our lack of recent exposure to common viruses, making us vulnerable to their return.

In hospitals across the country, physicians are adjusting protocols that for decades reflected a predictable cycle of illnesses that would come and go when schools closed or the weather changed.

“You would see a child with a febrile illness, and think, ‘What time of the year is it?’ ” said Peter Hotez, a molecular virologist and dean for the National School of Tropical Medicine at Baylor College of Medicine in Houston.

For years, Theresa Barton, head of pediatric infectious diseases at University Health in San Antonio, has routinely championed the flu vaccine each fall and relaxed her advocacy by March and April, when the flu fizzled out. The new shift in seasonality, with flu cases rising last summer and then again this spring, made her rethink.

“You are like, ‘Oh man!’ in clinics. ‘Let’s get your flu shot,’ ” Barton said.