Abstract
For the purpose of epidemiological study, a disease can be defined either by creating a definition and regarding as cases the subjects that fit it or by accepting as cases those subjects that have been so diagnosed by a doctor. To complicate matters, many disease definitions have changed and continue to change, sometimes even in a major way, with advances in biological and medical knowledge. ‘Measuring health and disease’ points out that three elements are always needed to measure the occurrence of a disease in a population: the number of cases of the disease, the number of people in the population, and an indication of time.
