Read the excerpt The Time Traveler's Guide to Elizabethan England from by Ian Mortimer. Elizabethans do not understand infection and contagion as we do. It is not that they are completely ignorant as to how illnesses spread—physicians believe they know perfectly well—it is rather that their understanding is very different from ours. The principal ideas underpinning most Elizabethan medical thinking come from Galen, who lived in the second century A.D. Physicians will cite him as an unquestionable authority when they explain to you that your health depends on a balance of the four humors: yellow bile or choler, black bile, phlegm, and blood. If there is too much choler in your body, you will grow choleric; too much blood and you will be sanguine; too much phlegm and you will be phlegmatic; and too much black bile makes you melancholic. It is from these imbalances that sickness arises. Which sentence best shows the central idea of the excerpt?