Causes and pathogenesis of psychiatric disorders is poorly understood. Infections by viruses or other agents may disturb neurotransmitters and elicit behavioral abnormalities, and induce long lasting immune reactions, referred to as mild encephalitis (ME). New findings (pathology, biochemistry, imaging) in schizophrenia and bipolar psychoses are compatible with ME hypothesis. In Chorea Sydenham and PANDAS syndrome autoimmune ME seems to explain anxiety-compulsive-hyperactivity symptoms. Add-on-therapy with Cox-II-blockers or valacyclovir improved acute schizophrenia, CSF filtration some cases of therapy resistant psychoses.