Identify everyone born in Denmark between 1985-2002. Identify those treated "in the primary care setting" for an infection. Identify those diagnosed with schizophrenia and affective disorders. Look-see whether there is an overlap between infection or treated infection and schizophrenia / affective disorders. Report results.
That's basically the study published by Köhler and colleagues  (a name that has appeared on this blog before) who concluded that: "Infections treated with anti-infective agents and particularly infections requiring hospitalizations were associated with increased risks of schizophrenia and affective disorders, which may be mediated by effects of infections/inflammation on the brain, alterations of the microbiome, genetics, or other environmental factors."
Mention of Denmark as being the source material for such a discovery means, yet again, that those various Scandinavian population registries are proving their 'big data' scientific worth. Indeed, such resources really do put countries such as Blighty to shame insofar as tracking the health of the Nation and at the same, providing science with lots and lots of research nuggets.
Taking into account various "confounders", authors report that alongside a possible connection between infection or treatment for infection being 'associated' with schizophrenia / affective disorders there was "a dose-response and temporal relationship" further substantiating their findings. Indeed, those who had to be hospitalised for infection (a serious infection then) were at much greater risk of subsequently being diagnosed with schizophrenia or affective disorders. This tallies with other research on this topic (see here).
One more thing: "The excess risk was primarily driven by infections treated with antibiotics, whereas infections treated with antivirals, antimycotics, and antiparasitic agents were not significant after mutual adjustment." Interesting. This suggests that bacterial infections or their treatments, e.g. antibiotics seem to be important factors in relation to the subsequent risk of schizophrenia and/or affective disorders. Again, this is not the first time that this point has been raised (see here and see here) and provides some corroborative evidence as to why the microbiome was mentioned by authors in relation to possible mechanisms to account for their findings.
I could start talking about how this research is further evidence for a role for the immune system and/or inflammatory processes in relation to psychiatric labels (see here for example) but you've probably heard it all before I'm sure. What we are also starting to understand with some confidence, is that infection and the effects of it's treatment might go way beyond just the somatic...
 Köhler O. et al. Infections and exposure to anti-infective agents and the risk of severe mental disorders: a nationwide study. Acta Psychiatr Scand. 2016 Nov 21.
Köhler O, Petersen L, Mors O, Mortensen PB, Yolken RH, Gasse C, & Benros ME (2016). Infections and exposure to anti-infective agents and the risk of severe mental disorders: a nationwide study. Acta psychiatrica Scandinavica PMID: 27870529