Infectious Causation of Disease: An Evolutionary Perspective

Abstract

Over the past two centuries, diseases have been separated into three categories: infectious diseases, genetic diseases, and diseases caused by too much or too little of some noninfectious environmental constituent. At the end of the 19th century, the most rapid development was in the first of these categories; within three decades after the first cause-effect linkage of a bacterium to a disease, most of the bacterial causes of common acute infectious diseases had been identified. This rapid progress can be attributed in large part to Koch's postulates, a rigorous systematic approach to identification of microbes as causes of disease. Koch's postulates were useful because they could generate conclusive evidence of infectious causation, particularly when (1) the causative organisms could be isolated and experimentally transmitted, and (2) symptoms occurred soon after the onset of infection in a high proportion of infected individuals. While guiding researchers down one path, however, the postulates directed them away from alternative paths: researchers attempting to document infectious causation were guided away from diseases that had little chance of fulfilling the postulates, even though they might have been infectious.