Epidemiological characteristics of foodborne diseases and pathogen spectrum changes in the "post-pandemic" era in Kunshan, 2021-2025
This study analyzes foodborne disease surveillance data from Kunshan (2021–2025) to reveal a significant post-pandemic rebound in 2025 driven by a surge in norovirus infections, which has displaced diarrheagenic *E. coli* as the dominant pathogen, while identifying summer season, young adulthood (30–40 years), and home dining as key independent risk factors.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Every year, millions of people around the world fall ill from food they have eaten. These illnesses, known as foodborne diseases, are caused by tiny invaders like bacteria and viruses that contaminate our meals. For decades, health officials have tracked these outbreaks, usually finding that bacteria such as Salmonella are the most common culprits. However, the world changed dramatically during the global pandemic. When people stayed home, washed their hands more often, and avoided crowds, the spread of many infectious diseases slowed down. Scientists now worry that as society returned to normal, these diseases might be bouncing back with a vengeance. This idea, often called "immune debt," suggests that because people were protected from germs for so long, their bodies may have forgotten how to fight them off, leaving them more vulnerable when exposure returns. Understanding whether these diseases are truly returning or if we are just seeing them more clearly due to better testing tools is a critical question for public health.
In the city of Kunshan, located in a bustling region of China, researchers set out to answer this question by looking at foodborne illness data from 2021 to 2025. They gathered information on thousands of cases reported to local hospitals, tracking who got sick, what they ate, where they ate it, and what specific germ was found in their stool samples. The team was particularly interested in the year 2025, wondering if the patterns of illness had shifted after the pandemic restrictions were lifted. They wanted to know if the types of germs causing sickness had changed, and if the rise in cases was a real surge in infections or simply a result of hospitals becoming much better at detecting them.
The results revealed a striking shift in the landscape of foodborne illness. For years, bacteria like diarrheagenic E. coli had been the most frequently detected cause of these infections. But in 2025, a virus called Norovirus took the lead, becoming the dominant pathogen. While the detection rate for the bacterial culprit steadily declined over the five years, the detection rate for Norovirus more than doubled, jumping from a low point in 2022 to a peak in 2025. The researchers found that nearly one in four people who submitted a sample in 2025 tested positive for a pathogen, a significant increase compared to previous years. This surge was not limited to just one group; it affected people of all ages, but it was most pronounced among adults between 30 and 40 years old.
The study also uncovered surprising details about where these infections were happening. While many people assume that foodborne outbreaks are mostly linked to restaurants or public cafeterias, the data showed that eating at home was actually a major risk factor. In fact, people who ate at home were more likely to test positive for a pathogen than those who ate at catering services. This finding suggests that the transmission of these germs, especially Norovirus, might happen more easily between family members in a household setting rather than through commercial food service. The researchers noted that Norovirus is highly contagious and can spread through contact with an infected person, even if that person shows no symptoms, which could explain why home environments became hotspots for infection.
Seasonal patterns also played a crucial role. The summer months, particularly July, saw the highest number of cases. This aligns with the fact that warmer temperatures can help bacteria and viruses survive and spread more easily. The study confirmed that summer is the peak season for these illnesses, with the risk of infection being more than twice as high during these months compared to spring. The researchers also looked at the specific symptoms of those infected. People with Norovirus were much more likely to experience vomiting and watery stools, whereas those with bacterial infections were more likely to have fevers. This distinction helps doctors and public health officials identify the likely cause of an outbreak based on the symptoms alone.
A major challenge for the researchers was determining whether the sudden rise in Norovirus cases in 2025 was a true epidemic or just an artifact of improved testing technology. Over the five-year period, hospitals upgraded their equipment to use more sensitive methods that could detect the virus more easily. To address this, the team performed a careful analysis to estimate how much of the increase was due to better tools versus a real increase in infections. They ran several scenarios, assuming that in earlier years, the tests might have missed a significant number of cases. Even under the most conservative assumptions, where they assumed the old tests missed up to 40 percent of cases, the data still showed that the number of Norovirus infections in 2025 was significantly higher than in previous years. This confirmed that the surge was a real phenomenon, not just a result of better detection.
The study also examined why the year 2024 stood out as an anomaly. That year saw a temporary dip in the number of positive cases, creating a "U-shaped" pattern when viewed alongside the rise in 2025. The researchers proposed several possibilities for this dip, including extreme heat that might have suppressed bacterial growth or a temporary pause in the circulation of specific virus strains. While they could not pinpoint the exact cause, the fluctuation highlighted the complex and dynamic nature of how these diseases move through a population.
Ultimately, the findings from Kunshan paint a clear picture of a changing public health landscape. The burden of foodborne disease remains high, but the primary threat has shifted from bacteria to viruses. The "post-pandemic" era has seen a rebound in Norovirus infections, driven by a combination of factors including increased social activity, the nature of the virus itself, and perhaps a population that has become more susceptible due to a lack of recent exposure. The researchers suggest that future prevention efforts need to adapt to this new reality. This includes focusing health education on the risks of eating at home, targeting specific age groups like adults in their 30s and 40s, and improving surveillance to distinguish between infections spread through food and those spread through contact with people. By understanding these shifts, communities can better protect themselves against the invisible threats that lurk in their daily lives.
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