control health problems (adapted from Porta and International Epidemiological Association, 2008). Health states or events usually refer to infection, illness, disability, or death but may equally be used to refer to a positive outcome (e.g. survival). Epidemiological studies describe the distribution of these health outcomes in terms of frequency and pattern. The frequency is the number of occurrences of an outcome within a given time period, and the pattern refers to the occurrence of the outcome by time, place and personal or population characteristics. Determinants infl uence the frequency and pattern of health outcomes and are known as risk factors or protective factors, depending on whether they result in a negative or positive health outcome respectively. Epidemiological research also involves the testing of preventive interventions (e.g. vaccines, improved hygiene) and therapeutic interventions (e.g. medicines, surgery) to improve health and survival. An intervention may be evaluated either under ideal (research-controlled) conditions to assess its effi cacy or through a routine delivery system to assess its effectiveness.
After collecting epidemiological evidence, its application to improve health is a natural progression. Identifi cation of risk factors and protective interventions, and quantifi cation of their effects are key to informing action. Knowledge of the distribution and time-trends of outcomes, risk factors, and intervention coverage may be used for advocacy, for health promotion, and to inform public health policy and practice.