Both epidemiological and clinical studies support an association between infections and autoimmune diseases with mental disorders. It has been suggested that infections and autoimmune diseases can increase the risk of mental disorders by inducing neuroinflammation potentially through immunopathological mechanisms. These mechanisms might increase the permeability of the blood–brain barrier leaving the brain more vulnerable to circulating brain-reactive autoantibodies and other immune components which can cause psychiatric symptoms. However, the association between infections and autoimmune diseases with mental disorders has rather consistently been shown to be bidirectional indicating that some of the associations could be partly explained by confounding factors as BMI and smoking. Nonetheless, dose–response associations have been found for autoimmune diseases and particularly for infections regarding the risk of development of mental disorders, and risks of mental disorders are further increased with the temporal proximity of the diagnosis, and if causal, it could show to be important risk factors for mental disorders. Future longitudinal studies are needed with measurements from multiple biological samples, ranging from material close to the brain, as cerebrospinal fluid (CSF), to analysis of blood, in order to clarify if the association between infections and autoimmune disorders and mental disorders is causal or rather an epiphenomenon. Nonetheless, screening for general medical conditions and particularly infections and autoimmune diseases in patients with onset of severe mental disorders, preferably based on CSF in combination with serum, is a key in order to improve the health condition of individuals with mental disorders. Continued research in the field is important since an increased understanding of the etiology of mental disorders can prompt a range of new treatment options in psychiatry.