This document defines and discusses various concepts related to disease transmission and epidemiology. It defines infectious disease as a clinically evident illness in mammals caused by pathogens that can be transmitted between individuals. Contagious and communicable diseases are transmitted through direct or indirect contact between people or animals. Disease frequency refers to the number of cases in a population, while distribution describes who is affected, when, and where. Disease determinants are factors that cause disease occurrence and are divided into primary, such as biological, physical and chemical causes, and secondary host and environmental factors that enable disease progression.