At the close of the 19th century, the germ theory had generated a new understanding of the causes of acute infectious diseases and revealed new directions for study. This understanding contributed to the greatest improvements in health in the history of medicine. At the end of the 20th century, the second stage of this disciplinary development is occurring. The old germ theory is being expanded into a new germ theory, which, by integrated the full spectrum of biologic disciplines. This new germ theory is emphasizing how environments and human activities influence the characteristics of infectious agents and the broader role of infection as a cause of chronic diseases.