As civilizations developed and human and domesticated animal population density increased, protozoan, bacterial and viral disease-causing organisms had an expanded field for their propagation. This increase was particularly rapid and occurred at a very early date in Mesopotamia. Many closely related civilizations developed in Mesopotamia, including the Sumerians. Even in the early Sumerian civilization, medicine had developed considerably. Much of our knowledge of their medical practices comes from cuneiform clay tablets, many of which are prescriptions for medicine. (c) 2002 Prous Science. All rights reserved.