An institution that provides care and shelter for people with mental, emotional, and personality disorders, sometimes severe enough to require custodial care. Mental hospitals have a long tradition, dating back at least to Pergamon in Asia Minor (350 BCE). Until the middle of the 19th century, there was almost no effective treatment for most of the inmates of such hospitals. Among their problems and challenges, an important one was the maintenance of good hygiene and sanitation. Influenced by the philosophy of deinstitutionalization, large custodial mental hospitals have been closed in many countries since the 1970s, and their patients removed to smaller residences or discharged to community care, despite the fact that such care often is inadequate. Cross infection remains a problem in institutions for the mentally retarded.