“Immunity debt,” a theory to explain the global surge in non-covid infections since pandemic restrictions were lifted, is increasingly being challenged by emerging evidence. Nick Tsergas reports Mycoplasma pneumoniae is a bacterial infection not known to cause widespread hospital admissions. “I can count on my two hands the number of times I’d ever seen mycoplasma pneumoniae before 2023,” says Samira Jeimy, clinical immunologist at the University of Western Ontario. “All of a sudden I feel like everybody has it.”1 Over the past three years similar reports have circulated of rising bacterial infections, flare-ups of old viruses becoming more common, and children landing in hospital with diseases not usually seen in young, healthy people. One explanation offered by public health leaders has been “immunity debt”2—the idea that precautions taken in the covid pandemic suppressed routine exposures to circulating pathogens, leaving people more vulnerable to them when restrictions were lifted. The theory landed in the public consciousness at the right moment. A simple idea that sounded like science, it soothed a public seeking answers just as the world was returning to a semblance of normality. And it served a policy function, allowing governments to focus on economic recovery. But its explanatory power has faded as the number of non-covid infections has kept rising each year. A 2024 analysis by the US Centers for Disease Control and Prevention3 found that invasive group A strep infections saw their most dramatic year-on-year increase from 2021 to 2022, well after most precautions had been lifted in the US. Rates have been abnormally high since then, raising questions about what might be behind the trend. A growing number of scientists believe that the SARS-CoV-2 virus may instead be subtly altering our immune systems. If correct, their hypothesis will change how we understand everything …