Donne’s intestinal treasure has not entered the pantheon of great metaphors. But the idea of turning disease into philosophical currency has inspired Susan Wise Bauer’s “The Great Shadow.” Ms. Bauer, who taught American literature at the College of William and Mary and is a prominent advocate of classical education, briskly surveys thousands of years of health and sickness, mostly of the infectious variety: that is, disease caused by invading organisms, usually microscopic. Plague, typhus and smallpox (all of which stalked Donne’s England) loom large, but typhoid, influenza, polio, tuberculosis, dysentery, scarlet fever, cholera, measles and malaria round out Ms. Bauer’s truly gruesome cast of characters.