Infections tied to higher risk of brain and mental health conditions for up to two years, study finds

A study of more than one million matched patient pairs found that adults hospitalised with serious infections were more likely to develop psychiatric and neurological conditions over the next two years, highlighting potential long-term healthcare needs and informing future pandemic preparedness.

People admitted to hospital with serious infections may face a higher risk of developing a range of brain and mental health conditions for up to two years after their illness, according to a large study led by the University of Oxford and published in JAMA Psychiatry.

The findings matter because they suggest that severe infections could leave lasting neurological and psychiatric effects, particularly in adults and older people. Researchers say the results could help health systems anticipate long-term healthcare needs after future infectious disease outbreaks and pandemics.

Large study maps long-term risks after hospitalised infections

The study, supported by the National Institute for Health and Care Research (NIHR) Biomedical Research Centre: Oxford Health, analysed electronic health records from 62 healthcare organisations in the United States. Researchers examined more than one million matched pairs of patients, comparing people hospitalised with infections affecting organs such as the lungs, urinary tract, skin, liver, heart, gastrointestinal system and brain with similar patients admitted for non-infectious reasons.

The analysis found that hospitalised infections were followed by increased risks of most of the 14 psychiatric and neurological outcomes examined. These included anxiety disorders, mood disorders, psychotic disorders, insomnia, cognitive problems, dementia, seizures, stroke, brain haemorrhage and nerve or muscle disorders.

Cognitive problems, which include memory and thinking difficulties, showed the largest absolute increase after infection, indicating the greatest additional burden among the conditions studied.

Encephalitis showed the strongest relative increase

The strongest relative increase was observed for encephalitis.  Encephalitis is inflammation of the brain. It can affect brain function and is one of the neurological conditions evaluated in this study. According to the study, people hospitalised with an infection were five times more likely to develop encephalitis than those admitted for non-infectious reasons.

However, the researchers emphasised that the greatest overall burden came from cognitive problems because they accounted for the largest absolute increase after infection.

The study also found that risks varied by age. While elevated risks were observed across all age groups, including children, they were generally lower in children than in adults. Older adults experienced the highest risks.

Different infections linked to different outcomes

Researchers found that different types of infections were associated with distinct patterns of later neurological and psychiatric conditions.

Infections directly involving the brain, such as infectious encephalitis, were among the strongest predictors of subsequent disorders.

Lead author Max Taquet, Associate Professor in the Department of Psychiatry at the University of Oxford and NIHR Academic Clinical Lecturer, said, “These findings provide the first map linking infections of different parts of the body with neurological and psychiatric consequences. This map will help prepare for future pandemics and outbreaks, and provides key information for studies aiming to understand the mechanisms by which an infection might have consequences in the brain.”

Study points to several possible biological pathways

The researchers said the findings suggest infections may influence brain health through multiple mechanisms rather than a single pathway.

Possible mechanisms include direct injury to the brain, inflammation, disruption of the blood-brain barrier, vascular injury, immune activation, or indirect effects associated with severe illness and hospitalisation.

The researchers stressed that the study does not establish that infections directly cause these later neurological or psychiatric conditions. Instead, it identifies where risks appear to be greatest and where future mechanistic research should focus.

Senior author Professor Paul Harrison, Professor of Psychiatry at the University of Oxford, said, “Our findings highlight the need to understand how and why many infections seem to increase the risk of many psychiatric disorders. In this way potential new treatments may be identified.”

Findings could inform future pandemic preparedness

The researchers said the results have implications for pandemic preparedness because health systems often have limited evidence on which long-term neurological and psychiatric complications may follow newly emerging infectious diseases.

The study suggests that the body system affected by an infection could serve as an early indicator of the types of post-infectious brain and mental health conditions that may emerge, allowing healthcare systems to better plan long-term care.

The researchers also acknowledged several limitations. The analysis relied on routinely collected electronic health records, which may miss or misclassify diagnoses. The study also included only infections severe enough to require hospitalisation, meaning the findings may not apply to milder infections managed in the community.

Although the analyses adjusted for many differences between patients, residual confounding cannot be excluded, and the study does not establish a cause-and-effect relationship between infections and later psychiatric or neurological conditions.