Throughout history, humankind's working theories regarding the cause of infectious disease have shifted drastically, as cultures developed their philosophic, religious, and scientific beliefs. Plagues that were originally attributed to the wrath of the gods were later described as having nothing to do with the gods, though the cause continued to be a mystery. As centuries passed, medical and religious theorists proposed reasons such as poor air quality or the configuration of the planets as causes for the spread of disease. In every instance, in order to understand the origin of a disease...