Statistical codes used in analysis.

BackgroundSevere infections have been linked to an increased risk of dementia, but both conditions often coexist with other illnesses that may confound this association. Using nationwide Finnish health registry data, we examined the role of noninfectious mental and physical illnesses in the association between severe infections and dementia.Methods and findingsThis register-based study included 62,555 individuals aged 65 or older in Finland in 2016 who were diagnosed with late-onset dementia between 2017 and 2020 and 312,772 dementia-free controls matched for year of birth, sex, and the follow-up period. Analyses were adjusted for education, marital status, employment, and area of residence, with age and sex accounted for through the matched conditional design and analysis. Applying a 1-year lag period, we identified 29 hospital-treated diseases that occurred 1–21 years before dementia diagnosis in cases (or index date in controls), had a prevalence of ≥ 1% prior to dementia, and were robustly associated with increased dementia risk (confounder-adjusted rate ratio ≥ 1.20, p p p p p ConclusionsThis nationwide Finnish study identified several mental and physical diseases that are associated with an increased risk of dementia and showed that the increased incidence of dementia among individuals with severe infections is not attributable to these comorbid conditions. These results support the role of severe infections as independent risk factors for dementia.