The immune system is linked to an increasing number of medical diseases, including lately also severe mental disorders. Utilizing the nationwide registers and biobanks in Denmark we have consistently displayed that infections and autoimmune diseases increases the risk of developing severe mental disorders in a dose-response relationship, where the risk of severe mental disorders particularly increases with the amount of infections exposed to. It is a broad range of infections and autoimmune diseases that are associated with an increased risk of severe mental disorders, with the more severe infections increasing the risk the most and the risk of severe mental disorders is particularly increased with the temporal proximity to the infection. Furthermore, we have shown that the genetics for psychosis does not explain the association with infections but that the genetics and infections seem to be additive risk factors for psychosis. Moreover, at diagnosis there are elevated levels of inflammatory markers in the blood and studies on the cerebrospinal fluid surrounding the brain have shown some evidence for elevated immune markers in the brain and signs of disrupted blood-brain barrier in some of the patients, making them more vulnerable to potential detrimental effects of immune components. Our meta-analyses of randomized clinical trials have shown that anti-inflammatory treatment seems to be effective for depression and depressive symptoms and also for psychotic disorders. However, studies are still lacking on subgroups that would be most likely to respond to anti-inflammatory add-on treatment.