The fifth century B.c.: Blood, phlegm, choleric/yellow bile, and melancholy/black bile make up the human constitution, and illness occurs when they are imbalanced. Although humoralism was discredited in the 19th century, Galen's extensive writings made it the dominant thinking for two millennia.Galen also supported Hippocrates' view that reason and clinical observation—not —should be used to treat disease. Hippocrates, called the father of modern medicine, remains a central figure in medicine because of the Hippocratic Oath, which spells out the ethical and code of behavior by which physicians must abide.Today's medical advances would not be possible without the founders of medicine: Galen and Hippocrates.N E Galen was in Rome when the Antonine Plague struck in A.D. 166. Although he only wrote ption of its symptoms is detailed enough to identify it later as smallpox.