More than a century ago, physicians began to reconceptualize disease. Rather than a discrete and uniform insult from an invasive process, disease might be considered the truly unique interaction between a perturbing phenomenon and an individual. That philosophical advance has been slow to inform nosology. As a result, concussion scholars spent much of the twentieth century fruitlessly working to characterize the clinical and pathological nature of CBI as a homogeneous disorder. It is not. This chapter attempts to summarize the literally innumerable variables that distinguish one CBI from another. That perspective, and the empirical evidence supporting it, seems extremely useful in rectifying and reconciling the diverse opinions about concussion. Respect for and attention to human individuality liberates scholars and practitioners from the straitjacket of nineteenth-century medical thought, and opens the door to what is sometimes called precision medicine.