Infectious diseases arise from the presence of a pathogenic biological organism or its toxic products in a susceptible host. They differ from non-infectious diseases because of the mechanisms by which pathogens are transmitted as well as the population dynamics of disease occurrence transmission. Infectious disease epidemiology is the study of factors contributing to our understanding of why infections emerge and spread and how they may be prevented and controlled. Epidemiological methods are used to detect infectious pathogens, determine disease causality, understand pathogenesis and the natural history of infections as well as ways to devise effective interventions for their prevention and control. For example, through surveillance, case control and cohort studies, it is possible to detect disease outbreaks and identify causative infectious agents as well as factors contributing to propagation. Increasingly, molecular methods are also used to inform infectious disease transmission dynamic. The appearance of new infectious diseases and resurgence of old ones, often in new and drug-resistant forms, brings renewed urgency to infectious disease epidemiology.