From infancy to old age, nationwide China study maps how serious lung infections change across life stages
Analysis of 695,142 hospitalized patients identifies eight life-stage pathogen profiles, three co-detection networks, and marked male predominance in bacterial and fungal infections
Science China Press
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This graphic summarizes a nationwide analysis of 695,142 bronchoalveolar lavage fluid samples from hospitalized patients with lower respiratory infections in China. It shows eight life-stage pathogen profiles, three characteristic co-detection networks built from 124 significant pathogen pairs, and sex-based differences in hospitalization patterns, including higher male representation for several bacterial and fungal pathogens.
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BEIJING - Serious lower respiratory infections do not follow the same pathogen rules at every age. A nationwide study in China has now shown that the microbes most often detected in hospitalized patients shift dramatically from infancy to old age, and that these shifts are accompanied by distinct co-detection networks and clear sex disparities.
The study, published in Science Bulletin, analyzed 695,142 bronchoalveolar lavage fluid samples from hospitalized patients with lower respiratory infections collected between January 2022 and May 2025. The samples came from 4,758 hospitals in 306 cities across all 31 provincial-level regions of mainland China. Using targeted next-generation sequencing, the researchers tested all samples for 28 common respiratory pathogens under a unified workflow, making this one of the largest standardized pathogen-mapping studies of hospitalized lower respiratory infections to date.
At least one of the 28 target pathogens was detected in 82.09% of samples. Overall, the most prevalent pathogens were Mycoplasma pneumoniae, Haemophilus influenzae, and Klebsiella pneumoniae. But the biggest message from the study was that different life stages faced very different pathogen landscapes. Instead of relying on broad conventional age brackets, the team used real-world pathogen data to define eight consecutive life-stage groups: infant, toddler, child, adolescent, young adult, middle-aged, elderly, and octogenarian.
The contrasts were striking. In infants, Pneumocystis jirovecii, rhinovirus, and respiratory syncytial virus together accounted for 62.59% of hospitalizations. In toddlers, children, and adolescents, Mycoplasma pneumoniae became the dominant detected pathogen, reaching 66.02% in children. As age increased, the spectrum shifted toward bacteria, with Klebsiella pneumoniae, Acinetobacter baumannii, and Pseudomonas aeruginosa becoming more prominent in the oldest patients. In other words, the main pathogens linked to hospitalization were not the same across the lifespan.
The study also found that pathogens did not simply appear one by one. Among 378 possible pathogen pairs, 124 showed significant positive co-detection relationships. These pairs clustered into three characteristic networks: a pediatric-associated network, a pan-age network dominated by fungi and certain viral co-detections, and an elderly-associated network. Nearly 80% of observed co-detections occurred within, rather than between, these networks. This pattern suggests that when one pathogen is detected, clinicians may need to pay closer attention to a predictable set of co-detected partners rather than treating co-detection as random.
Sex differences were another major finding. Male hospitalized cases outnumbered female cases by 1.58 to 1, corresponding to 156,288 excess male hospitalizations in the study population. This imbalance was driven mainly by bacterial and fungal pathogens rather than by viruses. Bacteria accounted for 48.14% of the excess male hospitalizations, and fungi accounted for 17.89%. Pathogens such as Klebsiella pneumoniae, Pneumocystis jirovecii, and Acinetobacter baumannii were detected more frequently in male patients. Age mattered here as well: males were overrepresented among hospitalized patients at ages 0 to 4 years, the gap narrowed between ages 5 and 35 years, and then widened again after age 35.
Together, these findings provide a more detailed picture of how hospitalized lower respiratory infections vary across life stages and sexes. The researchers say the results may help public health teams refine age- and sex-stratified prevention strategies, support developers in designing life-stage-specific diagnostic panels, and assist clinicians in anticipating co-detection patterns in hospitalized patients.
To support real-world use, the team also developed an interactive online platform that allows clinicians and public health practitioners to query age- and sex-stratified pathogen detection data and co-detection patterns from hospitalized lower respiratory infection cases nationwide: https://psi.tracepatho.com/
The authors note that the study reflects hospitalized patients who underwent bronchoalveolar lavage fluid sampling and targeted sequencing in routine care. As a result, the dataset may overrepresent more severe or diagnostically complex cases and should not be taken as a direct proxy for all lower respiratory infections seen in outpatient settings.
The study was led by Yamin Sun, together with Min Wang, Tian Qin, Jue Liu, Jiale Yuan, Pei Li, Xinchang Lun, Zhigang Wang, Linghang Wang, Rui Song, Xi Wang, Ronghua Jin, and Jianguo Xu. Ronghua Jin and Jianguo Xu are the co-corresponding authors. The work was supported by the National Key Research and Development Program of China and the Tengfei Initiative of the National Key Laboratory of Intelligent Tracking and Forecasting for Infectious Diseases.
About Beijing Ditan Hospital, Capital Medical University:
Beijing Ditan Hospital, Capital Medical University, established in 1946, is a top-tier comprehensive hospital specializing in infectious diseases. Beijing Ditan Hospital is the National Center for Infectious Diseases (Beijing) and is also a China Regional Medical Center. With major clinical, teaching, and research responsibilities in infectious diseases and related disciplines, the hospital plays a leading role in the diagnosis, treatment, and translational research of infectious diseases in China.
About the National Key Laboratory of Intelligent Tracking and Forecasting for Infectious Diseases:
The National Key Laboratory of Intelligent Tracking and Forecasting for Infectious Diseases is a national research platform relying on China CDC and co-established with Nankai University and Beijing Ditan Hospital, Capital Medical University. The laboratory integrates expertise in pathogen tracing, outbreak surveillance, early warning, data science, and intelligent decision-making to support the prevention and control of infectious diseases. Its work focuses on building advanced technologies and interdisciplinary research capacity for infectious disease monitoring, prediction, and public health response.
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