title: The Association between Infections and General Cognitive Ability in Young Men - A Nationwide Study.
abstract: BACKGROUND
  Infections and activated immune responses can affect the brain through several pathways that might also affect cognition. However, no large-scale study has previously investigated the effect of infections on the general cognitive ability in the general population.
  
  
  METHODS
  Danish nationwide registers were linked to establish a cohort of all 161,696 male conscripts during the years 2006-2012 who were tested for cognitive ability, which was based on logical, verbal, numerical and spatial reasoning at a mean age of 19.4 years. Test scores were converted to a mean of 100.00 and with a standard deviation (SD) of 15. Data were analyzed as a cohort study with severe infections requiring hospitalization as exposure using linear regression.
  
  
  RESULTS
  Adjusted effect sizes were calculated with non-exposure to severe infections as reference, ranging from 0.12 SD to 0.63 SD on general cognitive ability. A prior infection was associated with significantly lower cognitive ability by a mean of 1.76 (95%CI: -1.92 to -1.61; corresponding to 0.12 SD). The cognitive ability was affected the most by the temporal proximity of the last infection (P<0.001) and by the severity of infection measured by days of admission (P<0.001). The number of infections were associated with decreased cognitive ability in a dose-response relationship, and highest mean differences were found for ≥10 hospital contacts for infections (Mean: -5.54; 95%CI: -7.20 to -3.89; corresponding to 0.37 SD), and for ≥5 different types of infections (Mean: -9.44; 95%CI: -13.2 to -5.69; corresponding to 0.63 SD). Hospital contacts with infections had occurred in 35% of the individuals prior to conscription.
  
  
  CONCLUSIONS
  Independent of a wide range of possible confounders, significant associations between infections and cognitive ability were observed. Infections or related immune responses might directly affect the cognitive ability; however, associated heritable and environmental factors might also account for the lowered cognitive ability.
authors.affiliation: Mental Health Centre Copenhagen, University of Copenhagen, Faculty of Health Sciences, Bispebjerg Bakke 23, 2400 Copenhagen NV, Denmark; National Centre for Register-based Research, Aarhus University, Fuglesangs allé 4, 8210 Aarhus V, Denmark.
authors.forename: Michael Eriksen
authors.initials: ME
authors.lastname: Benros
authors.affiliation: Mental Health Centre Copenhagen, University of Copenhagen, Faculty of Health Sciences, Bispebjerg Bakke 23, 2400 Copenhagen NV, Denmark; The Lundbeck Foundation Initiative for Integrative Psychiatric Research, iPSYCH, Aarhus, Denmark.
authors.forename: Holger Jelling
authors.initials: HJ
authors.lastname: Sørensen
authors.affiliation: National Centre for Register-based Research, Aarhus University, Fuglesangs allé 4, 8210 Aarhus V, Denmark.
authors.forename: Philip Rising
authors.initials: PR
authors.lastname: Nielsen
authors.affiliation: Mental Health Centre Copenhagen, University of Copenhagen, Faculty of Health Sciences, Bispebjerg Bakke 23, 2400 Copenhagen NV, Denmark; The Lundbeck Foundation Initiative for Integrative Psychiatric Research, iPSYCH, Aarhus, Denmark.
authors.forename: Merete
authors.initials: M
authors.lastname: Nordentoft
authors.affiliation: National Centre for Register-based Research, Aarhus University, Fuglesangs allé 4, 8210 Aarhus V, Denmark; The Lundbeck Foundation Initiative for Integrative Psychiatric Research, iPSYCH, Aarhus, Denmark.
authors.forename: Preben Bo
authors.initials: PB
authors.lastname: Mortensen
authors.affiliation: National Centre for Register-based Research, Aarhus University, Fuglesangs allé 4, 8210 Aarhus V, Denmark.
authors.forename: Liselotte
authors.initials: L
authors.lastname: Petersen
mesh_terms: D000293:Adolescent; D001424:Bacterial Infections / Q000150:complications / Q000453:epidemiology* / Q000503:physiopathology / Q000523:psychology; D001921:Brain / Q000503:physiopathology; D003071:Cognition / Q000502:physiology; D003072:Cognition Disorders / Q000150:complications / Q000453:epidemiology* / Q000503:physiopathology / Q000523:psychology; D015331:Cohort Studies; D003718:Denmark / Q000453:epidemiology; D006801:Humans; D007361:Intelligence Tests; D016014:Linear Models; D008297:Male; D008889:Military Personnel; D009181:Mycoses / Q000150:complications / Q000453:epidemiology* / Q000503:physiopathology; D012042:Registries; D012107:Research Design; D014777:Virus Diseases / Q000150:complications / Q000453:epidemiology* / Q000503:physiopathology / Q000523:psychology; D055815:Young Adult