'Herman Melville is not well," one of the famously gloomy author's friends wrote in the 1850s. "Do not call him moody, he is ill." Melville's eyes, "tender as young sparrows," were so sensitive that he had a shaded porch built onto his house to spare them the full light of day. He also complained of severe back pain—a combination of symptoms that makes John J. Ross, a physician at Brigham and Women's Hospital in Boston, suspect an inflammatory autoimmune disease. (He suggests, however, that Melville also had bipolar disorder.)

This...