Psychiatric symptoms can be associated with several systemic and central nervous system infections and they can be the initial presenting symptoms, occurring in the absence of neurological symptoms in some disorders as in some cases of viral encephalitis. They could also be part of the clinical picture in other cases such as psychosis or mood symptoms secondary to brucellosis or toxoplasmosis. Late-onset neuropsychiatric complications may also occur several years following the infection such as in the case of subacute sclerosing panencephalitis due to measles. Some Infectious diseases may have possible etiological role for major psychiatric disorders, based on yet unconfirmed reports for viral infectious diseases (e.g. Influenza virus and HSV-1) which are thought to have risk for developing schizophrenia and psychosis. Neuropsychiatric adverse effects can occur due to drugs (e.g. mefloquine, interferon-alpha) that are used for treatment of infectious diseases. Psychiatric symptoms can also be reactivated resulting from chronic, complicated and serious infections such as HIV that can lead to depression, anxiety or adjustment disorders, although CNS involvement can also be a possible etiological factor. Patients suffering from primary and severe psychiatric disorders are at increased risk of contracting infection; that is mainly related to high risk behaviors in patients with mania or schizophrenia. It is also important to consider that the co-occurrence of psychiatric symptoms and infection can be incidental (i.e. infectious diseases can occur in psychiatric patients regardless of the above mentioned factors). Early identification of the underlying etiology for organic/secondary psychiatric symptoms is essential for appropriate intervention and early treatment of the primary condition that could be the etiology of psychiatric symptoms so as to avoid unnecessary long-term psychiatric treatment and to avoid complications of possible misdiagnosis or delayed diagnosis of the primary condition.