Over six weeks this spring, 23 children were admitted to a Tennessee hospital for treatment of parechovirus, a common virus that in rare cases can a pose a lethal threat to infants, according to a report released this week by the Centers for Disease Control and Prevention.

Twenty-one of the children have recovered without complications, but one was at risk for hearing loss and blood clots, the C.D.C. said, while another child experienced persistent seizures and was expected to suffer from severe developmental delay.

The children admitted to the Nashville hospital — Monroe Carell Jr. Children’s Hospital at Vanderbilt University — were between 5 days and 3 months old, and their illnesses were detected from April 12 to May 24, the C.D.C. said. The report described the infections as an “unusually large cluster.” Six more cases have been identified at the hospital at other times this year, a “peak in infections” compared with recent years, according to the report.

Thirteen of the patients were girls and 10 were boys, and all of them were previously healthy, the C.D.C. said.

Not long after this cluster, the C.D.C. alerted doctors this month that the type of parechovirus most associated with serious illness had been circulating nationally since May. It suggested parechovirus as a diagnosis to consider for babies with an unexplained fever or seizures.

Parechovirus is so common that most children have been infected with it by the time they reach kindergarten age, and its symptoms include a runny nose and sneezing — what we normally associate with the common cold.

But infants under 3 months, and particularly those less than a month old, are at greater risk for severe illness, according to the C.D.C.

There is no cure for parechovirus, but diagnoses can still govern how doctors manage the illness.

Experts say it is possible that the rise in cases comes from increased socializing after a period of lockdowns during which people were not exposed to common pathogens, which might have weakened their immune systems. But it is also possible that babies are simply being tested for parechovirus more often.

“Our ‘eyes’ have gotten better, therefore we are seeing more,” Dr. Kenneth Alexander, chief of infectious diseases at Nemours Children’s Hospital in Florida, told The New York Times this month.