The second serious problem for the normal function approach is the challenge of prevention. While some efforts at health promotion, such as exercise, straddle the border of medical responsibility, many preventive interventions (i.e., vaccines) are widely accepted as legitimate parts of medicine’s mission and are located squarely on the treatment side of the enhancement boundary. One of the ways to prevent disease is to strengthen the body’s ability to resist pathological changes before any diagnosable problem appears. But to the extent that prevention attempts to elevate bodily functions above the normal range for the individual (and, in some cases, even the species typical range), it seems to slide into what the normal function approach would call enhancement. If the normal function account is taken seriously as a biomedical boundary marker, how does one defend this kind of prevention? Conversely, if preventive interventions like these are acceptable in medicine, what can it mean to claim that researchers and clinicians should be “drawing the line” at enhancement?