As early as 4000 BC religions identified certain of their deities with healing. The temples of Saturn, and later of Asclepius in Asia Minor, were recognized as healing centres. Brahmanic hospitals were established in Sri Lanka as early as 431 BC, and King Asoka established a chain of hospitals in Hindustan about 230 BC. Around 100 BC the Romans established hospitals (valetudinaria) for the treatment of their sick and injured soldiers; their care was important because it was upon the integrity of the legions that the power of Rome was based.
It can be said, however, that the modern concept of a hospital dates from AD 331 when Constantine , having been converted to Christianity , abolished all pagan hospitals and thus created the opportunity for a new start. Until that time disease had isolated the sufferer from the community. The Christian tradition emphasized the close relationship of the sufferer to his fellow man, upon whom rested the obligation for care. Illness thus became a matter for the Christian church.
Religion continued to be the dominant influence in the establishment of hospitals during the Middle Ages . The growth of hospitals accelerated during the Crusades , which began at the end of the 11th century. Pestilence and disease were more potent enemies than the Saracens in defeating the crusaders. Military hospitals came into being along the traveled routes; the Knights Hospitalers of the Order of St. John in 1099 established in the Holy Land a hospital that could care for some 2,000 patients. It is said to have been especially concerned with eye disease, and may have been the first of the specialized hospitals. This order has survived through the centuries as the St. John's Ambulance Corps.
Throughout the Middle Ages, but notably in the 12th century, the number of hospitals grew rapidly in Europe. The Arabs established hospitals in Baghdad and Damascus and in Córdoba in Spain. Arab hospitals were notable for the fact that they admitted patients regardless of religious belief, race, or social order. The Hospital of the Holy Ghost, founded in 1145 at Montpellier in France, established a high reputation and later became one of the most important centres in Europe for the training of doctors. By far the greater number of hospitals established during the Middle Ages, however, were monastic institutions under the Benedictines, who are credited with having founded more than 2,000.