A cluster of 23 infants in Tennessee were diagnosed with a potentially severe childhood virus within a six-week span this spring — an unusually short amount of time for such a large number of cases, doctors reported Thursday.

The infants, ages 5 days to 3 months old, were infected with a type of parechovirus, according to a report published by the Centers for Disease Control and Prevention. This type of virus is not uncommon in children, but in babies younger than 6 months, it can be particularly dangerous, causing fevers, seizures and brain inflammation.

All of the infants developed a complication called parechovirus meningoencephalitis — an inflammation of the brain and brain lining — and were treated at Monroe Carell Jr. Children’s Hospital in Nashville. All but one were ultimately admitted to the hospital.

The CDC’s report comes several weeks after the agency issued a nationwide health alert about an uptick in cases of parechovirus meningoencephalitis. The alert said that a particular strain of the virus, called PeV-A3, was circulating in “multiple states.” The strain, the agency said in the alert, is most often associated with severe illness.

Among the cases in Tennessee, 21 of the infants fully recovered, but one was left with lasting seizures. Another appears to have developed hearing loss.

“What was surprising to us, and why we put this report together, is that we saw a higher than usual number of babies with this infection than we’ve seen in prior years,” said Dr. Ritu Banerjee, an author of the new report and a professor of pediatric infectious diseases at Vanderbilt University Medical Center in Nashville.

Nineteen such cases were diagnosed at the children’s hospital over five months in 2018, the first year testing for the virus was available, she said. Only seven cases were diagnosed from 2019 through 2021.

The low numbers in recent years are likely the result of pandemic lockdowns, Banerjee said.

The latest surge of 23 cases occurred during a six-week window, from April 12 to May 24. Six other cases were diagnosed at Monroe Carell Jr. Children’s Hospital this year, but outside of the narrow time frame included in the report.

It’s not entirely clear why doctors saw so many cases this spring; however, Banerjee suspects a relaxation of physical distancing played a huge role.

“Children were back in day care, back in school, going to camp,” she said. “Now the virus could circulate among many more vulnerable hosts.”

It’s another example of an uptick in unusual viral activity amid the Covid pandemic. Last summer, doctors noted an odd summer surge in respiratory syncytial virus, or RSV, in kids — a virus that’s more commonly seen in winter months.

Most of the 23 babies treated at the Tennessee hospital had become ill at home, and most had older siblings or were exposed to other children. One developed symptoms while still in the neonatal intensive care unit. The virus is spread through respiratory droplets and feces.

Initial symptoms in these cases included fever, fussiness and poor feeding, according to the report. Others also developed unusual sleepiness and congestion. Just one of the 23 developed seizures.

There is no specific treatment for parechovirus, no antiviral and no vaccine to prevent it. Doctors mainly help keep the babies hydrated and fever at bay as the virus works through their systems.

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Erika Edwards is a health and medical news writer and reporter for NBC News and “TODAY.”