Epidemiology: Incidence, prevalence & mortality rate

Epidemiology is the scientific study of the distribution, causes, and control of diseases in populations. It forms the basis for public health decisions and clinical research by identifying risk factors, tracking disease spread, and evaluating interventions.

Incidence

• Definition: Incidence refers to the number of new cases of a disease that develop in a specific population during a defined time period.
• Expressed as: A rate (e.g. per 1,000 or 100,000 people per year) or as a percentage of the population at risk.
• Use: Helps identify risk and causation; commonly used in cohort studies and trials.

Prevalence

• Definition: Prevalence is the proportion of individuals in a population who have a disease at a particular time.

Types:

• Point Prevalence: Proportion of individuals with the disease at a specific point in time.
• Period Prevalence: Proportion of individuals who had the disease at any point during a defined time period. This can be estimated using repeated cross-sectional studies.

Relation to disease duration:

• For chronic diseases, prevalence is typically higher than incidence because the disease persists over time.
• For short-duration illnesses (e.g. the common cold), incidence may exceed prevalence due to rapid resolution.

Mortality rate

• Definition: A type of incidence rate representing the number of deaths due to a specific disease in a population during a time period.
• Use: Essential for assessing disease severity and public health impact.

Morbidity rate

• Definition: The rate at which new non-fatal cases of a disease occur in a population at risk during a specific time.
• Use: Helps track the burden of disease and its impact on quality of life.

Median survival

• Definition: The length of time at which 50% of patients with a disease are expected to be alive.
• Use: Common in cancer epidemiology and survival analysis; often derived from Kaplan–Meier survival curves.