Decades passed following Jenner’s promotion of cowpox vaccination as an effective preventative for smallpox, and science slowly began to recognize and appreciate the connection between microorganisms and disease. The concept that bacteria, viruses, and other microorganisms can infect humans and produce specific diseases is so entrenched in our modern lives that this concept almost seems intuitive. In contrast, at the start of the 1800s, the concept that microbes could cause diseases was largely unknown and foreign to the prevailing beliefs. Ancient societies mainly attributed disease to supernatural causes, viewing illness as a punishment or tribulation imposed by gods and spirits. As civilizations advanced and became more sophisticated, emerging physicians and philosophers argued and postulated about the nature and causes of illness. While diseases were still mostly attributed to mystical entities, early Greek writing proposed that illnesses arose from an imbalance or corruption in the four humors: blood (sanguine), black bile (melancholic), yellow bile (choleric), and phlegm (phlegmatic) (Lagay 2002); these four humors also corresponded to the natural elements of air (blood), earth (black bile), fire (yellow bile), and water (phlegm). This idea that disease stemmed from real-world factors rather than supernatural causes, was an important conceptual advance even if it would take centuries before there was an actual scientific understanding of diseases. Ascribing disease to causes that were at least somewhat under human control meant that tangible therapies could be developed as an alternative to simply praying to some deity. Thus, adherents of the humoral theory could try to cure illnesses by re-establishing the correct equilibrium among these elements. For example, fevers (“hot blood”) were often treated by reducing “excess” blood using leeches or by making cuts in the patient. While mostly ineffective, and some perhaps even harmful, the humoral therapies at least encouraged medical practitioners to try something to help patients and provide relief for various ailments. This spirit of active intervention rather than passive acceptance of disease underlies all the progress that eventually led to modern medicine.