In spite of progress in biomedical research, we know little about the causes, prevention, and treatment of the numerous mental and neurological disorders that afflict up to 15% of all individuals. In the last decade, great advances have been made in the physiopathology of mental and neurological disorders, leading to at least a partial control of Parkinson's disease, epilepsy, certain psychoses, and anxiety syndromes. Despite the fact that an underlying specific neurotransmitter deficiency has been demonstrated in Alzheimer's and Parkinson's diseases, the immune dysfunction and viral hypotheses continue to be attractive for investigators dealing with these degenerative diseases of the aging brain, which afflict 10% of senior citizens. A retrospective epidemiologic study suggests that the encephalitis lethargica and parkinsonism were almost certainly caused by the 1918 influenza virus pandemics. It must be stressed that the etiopathogenesis of many mental disorders is not known, and this ignorance has led to several untenable neurophysiological and biochemical hypotheses. Epidemiologic investigations show a high prevalence of functional psychoses and organic mental disorders. Although many of them are conceptualized as biopsychosocial disorders, recent data indicate that the biological component appears more and more as a major etiologic factor. Among the various biological hypotheses, the viral and im munologic concept has become a significant one. In view of recent discoveries in virology and immunity, it becomes clear that viral and immunologic hypotheses should be inves tigated more systematically concerning the mechanisms of numerous mental and neu rological disorders.