This chapter introduces key foundational concepts in epidemiology essential for understanding disease dynamics and public health strategies. It begins with the principles of prevention, outlining the different levels of intervention aimed at reducing disease occurrence and impact. Measures of disease frequency are then explored, including incidence risk and incidence rate, which quantify the occurrence of new cases over time, and point prevalence and period prevalence, which reflect the proportion of a population affected at a specific point or over a defined period. Finally, the concept of herd immunity is discussed, emphasizing its role in protecting populations against infectious diseases through sufficient immunization coverage.