Epidemiology is the study of the distribution and determinants of diseases in populations. It is the key quantitative discipline that underpins public health, which is often defined as the organized efforts of society to prevent disease and promote health. Epidemiology, literally meaning “the study of what is upon the people”, is derived from the Greek epi ‘upon, among’, demos ‘people, district’, and logos ‘study, word, discourse’, suggesting that it applies only to human populations.However, the term is widely used in studies of zoological populations (veterinary epidemiology), although the term “epizoology” is available, and it has also been applied to studies of plant populations (botanical or plant disease epidemiology)