“Every children’s hospital that I’m aware of is absolutely swamped,” said Dr. Coleen Cunningham, the pediatrician in chief at Children’s Hospital of Orange County, a 334-bed facility in Southern California that is so full that children are being treated right in the emergency room as they wait for inpatient beds — sometimes for more than 24 hours.

R.S.V. is a common seasonal infection, and the vast majority of cases are very mild. But this year, the number of children falling ill — and seriously ill — is significantly greater than usual. Doctors suspect that those who would ordinarily have been exposed to R.S.V. over the last couple of years were insulated from it by social distancing measures and are now driving up the numbers.