3 Analytic epidemiology
Causation of ill health is difficult to determine. Apart from infectious diseases, most evidence relates to the risk associated with particular factors, rather than the direct causes of ill health. Even in the case of infectious diseases, it is not known why certain people succumb to them when other seemingly similar people do not. Only certain types of causative mechanism are amenable to investigation by epidemiological methods and few diseases can be said to have a single cause. Epidemiological evidence regarding disease causation is mainly circumstantial. So how can the information on persons, place and time help in the understanding of the causes of disease? An important model, originally developed for use with infectious diseases, but which has been adapted for much broader use in recent years, is the epidemiological triad.