From WikiLectures

Existing cases = prevalent cases. Prevalence is the proportion of individuals in a defined population that has the outcome (disease) under study at a defined instant (a point) in time ("point prevalence"); - Ranges between 0 and 1 (0%-100%). We use cross-sectional studies to obtain it. Achieved in intervals, cases are monitored in given time period (1 month, 1 year).