Not until the beginning of the 20th century did Europe's urban populations finally become self-sustaining: before then, constant immigration of healthy peasants from the countryside was necessary to make up for the constant deaths of city dwellers from crowd diseases.

F. 2005 Collapse: How Societies Choose to Fail or Succeed. PMID 3762700.

Originally trained in physiology Diamond's work is known for drawing from a variety of fields including anthropology ecology geography and evolutionary biology. He is currently Professor of Geography at the University of California Los Angeles.