Preventive care applies epidemiological and pathophysiological principles to interrupt disease processes before clinical manifestation, operating across three levels: primary prevention (averting disease onset), secondary prevention (detecting presymptomatic disease), and tertiary prevention (minimizing disability from established disease). Primary prevention targets modifiable risk factors and employs immunizations — vaccines stimulate adaptive immunity by presenting attenuated or inactivated antigens to antigen-presenting cells, which activate T-helper cells and B-lymphocytes to produce pathogen-specific antibodies and memory cells, providing long-term protection without causing disease. The CDC immunization schedule is guided by immunological principles: conjugate vaccines (PCV13, Hib) link polysaccharide antigens to protein carriers to elicit T-cell-dependent immune responses in infants whose immature immune systems cannot mount adequate responses to polysaccharides alone. Secondary prevention through screening follows principles established by Wilson and Jungner: the disease must have a detectable preclinical phase, an available and acceptable screening test, and an effective treatment that improves outcomes when applied early versus late. The NP applies the USPSTF grading system (A/B = recommended, C = offer selectively, D = recommend against, I = insufficient evidence) to guide evidence-based screening decisions. Cancer...