The main idea is measuring how often new cases occur in a population over a defined time period. This is incidence. It specifically tracks the risk of developing the disease, focusing on new occurrences rather than existing cases. Hydration, hematologic, and infection do not describe the rate at which new cases appear: infection is the process of becoming infected, hydration is fluid status, and hematologic relates to blood. Incidence can be expressed as cumulative incidence (the proportion of people at risk who develop the disease over a set period) or as an incidence rate (the number of new cases per person-time, such as per 1,000 or per 100,000 person-years). For example, if 50 new cases occur in a population of 10,000 people over a year, the cumulative incidence is 50/10,000 = 0.005, or 0.5% for that year.