Koopmans said a study her team did looking for antibodies in the blood of young children showed the impact of what she calls an “infection honeymoon.”

“You really see that children in the second year of the pandemic have far less antibodies to a set of common respiratory viruses. They just got less exposed,” she said.

Such factors may help explain the recent rash of unusual hepatitis cases in young children. Scientists investigating the cases think they may be caused, at least in part, by adenovirus type 41, because it has been found in a significant number of the affected children. The possibility is puzzling, because the virus hasn’t been seen to cause this type of illness in the past.

But some scientists theorize that this virus may have always been responsible for a portion of the small number of unexplained pediatric hepatitis cases that happen every year. Maybe, the thinking goes, there have been a lot more adenovirus type 41 infections over the past eight months because of increased susceptibility among children. That, in turn, could be making visible something that wasn’t spotted before.