Introduction Overview incidence and prevalence are methods of measuring disease frequency in a population with respect to time Incidence describes the amount of new disease cases in at-risk people over a certain time period at risk means those who are capable of developing the disease of interest do not already have the disease are not vaccinated against the disease have at-risk anatomy (patients without a prostate cannot get prostate cancer) incidence can be specified 2 ways # of new cases in a population/# of at-risk people in the population per unit time e.g., 9 cases of Kawasaki disease per 10,000 children per year # of new cases in a population/time spent at risk (person-time) person-time = number of people at risk x time spent at risk e.g., 9 cases of Kawasaki disease/10,000 child-years if we watched 10,000 at-risk people (children) for 1 year, 5,000 children for 2 years, or 1,000 children for 10 years, etc., we would see 9 new cases during that observation period Prevalence proportion of population that has a disease or risk factor at a specified point or period in time existing cases/total number of people in specific population at a particular time e.g., percent of the U.S. population with diabetes in 2018 indicates overall disease burden of population helpful for resource allocation prevalence = incidence x duration factors that change prevalence factors that increase prevalence include increase in new cases (increased incidence) improved quality of care → decreased mortality → longer duration improved diagnostic ability → higher incidence in-migration of cases or susceptible people out-migration of healthy people factors that decrease prevalence include high case-fatality rate → shorter duration (aggressive cancers) decrease in new cases (decreased incidence) could result from preventative efforts such as vaccination in-migration of healthy people out-migration of cases improved recovery/cure rate