One of the oldest intellectual traditions in clinical medicine is the concept that the natural history of a disease is a guide to treatment. Without a knowledge of what will happen if the patient is left alone, a clinician cannot wisely make the many decisions with which various modes of therapy are chosen, changed, combined, or withheld.
Until the diagnostic and therapeutic advances of the past century, clinicians could easily observe the natural history of disease. Disease itself had been regarded as a clinical entity—the fevers, cyanoses, and anginas noted at the bedside—and the course of most diseases could seldom be altered by the ineffectual agents then available for treatment. For doctors of the past, the main intellectual problem about natural history was not its observation but its interpretation. Because of post hoc ergo propter hoc reasoning and failure to recognize the self-limited duration of many acute ailments, doctors for