The main purpose of the volume Psychiatry and Biological Factors is to provide a comprehensive, state-of-the-art overview of the current research linked essentially to virus infections, immunity functions, and mental diseases. In recent years substantial advances have been registered in the physiopathol­ ogy of mental and neurological disorders. As a result, partial control of certain psychoses, anxiety syndromes, epilepsy, and Parkinson's disease is now possible. However, despite progress in biomedical research, numerous mental and neuro­ logical disorders afflict up to 15% of all individuals and little is known about the causes, prevention, and treatment of these diseases. Several epidemiological investigations demonstrated a high prevalence of functional psychoses and organic mental disorders, and recent data show that biological components appear as a major etiologic factor. In this respect it could be stressed that viral and immunologic hypotheses should be investigated seriously and systematically in relation to the mechanisms of several mental and neurologi­ cal diseases. Neuropsychiatric consequences of AIDS related to human immunodeficiency virus infection are now well documented. A variety of behavioral. symptoms and psychiatric syndromes with paranoid features are frequent concomitants of AIDS.