1 The proportion of individuals in a population who have disease at a specific point in time Number of current/existing case (old and new) of a specified disease at a point of time It help to estimate the burden of disease Identify potentially high-risk populations. They are essentially helpful to plan rehabilitation facilities, manpower needs, etc. (Number of current case of a specified disease at a point of time)/(estimated population at the same point of time) x 100 2 Measure the individual risk of disease; Useful in estimating the probability that an individual will be at ill at specific point in time