Dr. Elena Fuentes-Afflick is chief of pediatrics at Zuckerberg San Francisco General Hospital and Trauma Center and vice dean for academic affairs at the University of California, San Francisco School of Medicine. Stephen Morrissey, the interviewer, is the Executive Managing Editor of the Journal. E. Fuentes-Afflick. Applying a Subpopulation Lens to Population Health. N Engl J Med 2021;