Investigator-initiated active collection of public health related data from individual units in a population over a set period of time to assess the disease status within the population. This can be carried out using a sample of the population of interest or using a census of all units. Surveillance is usually aimed at assessing changes in disease status over time so generally repeated surveys are required when carrying out surveillance. Surveys of all units in the population can be used when the aim is to detect cases and facilitate eradication. Surveys are also frequently used to demonstrate freedom from disease.