Background
It has been debated whether specific infections may be causally linked to dementia development. The hypothesis has been principally directed towards Alzheimer’s disease (AD) pathology, prompted by findings that Aβ peptide, whose deposition is a hallmark of AD, has antimicrobial properties. Previous epidemiological population-based studies have reported that observed links between infection and dementia were more prominent for vascular dementia (VaD), whereas there was either a small or no increased AD risk. Similar findings have been reported for autoimmune disease, although links were overall close to null. Investigating the two exposures together gives insights into the involvement of the immune system in dementia risk. However, data on dementia subtypes in the previous studies predominantly relied on electronic health data with generally low or unknown validity. Therefore, it remains unknown whether links with infection are attributed to AD, VaD, or other dementias. It is also unknown whether the general lack of link with autoimmune disease could be modified by dementia subtypes. Further, evidence is lacking regarding young-onset dementias, with at least one study suggesting higher dementia risk in cases diagnosed at <60 years. The aim of this study was to investigate the association between infections, autoimmune diseases, and subsequent incidence of major dementia disorders.

Methods
We conducted a nationwide nested case-control study of dementia cases in the Danish Quality Database for Dementia (DanDem). This is a nationwide register of all dementia diagnoses given by memory clinics and are thus advantageous over previously used data on subtype diagnoses in our and other general health registries. Conditional logistic regression estimated incidence rate ratios for cases of AD, VaD, and other dementias, compared with controls, following exposure to infection and autoimmune disease (in the prior 35+ years).

Results
We identified all incident dementia cases between 2016 and 2018 and matched them to up to three controls on age and sex. Majority of cases were diagnosed with AD, followed by mixed dementia and VaD.

Conclusion
The results of this study will further knowledge on the role of the immune system for dementia risk and contribute with new data regarding the risk of different dementias.