Prevalence is one of two basic ways of describing the occurrence of disease in a population (see also incidence). Prevalence is the proportion of people in the entire population who are found to be with disease at a certain point in time (sometimes called a “cross section”), without regard to when they first got the disease. If the disease is irreversible but not fatal, the numerator of the prevalence is the cumulative sum of diseased persons up to that point in time in a fixed population. If the disease is reversible or treatable, or if it is eventually fatal, the prevalence doesn’t reflect the total number of people who have ever had the disease up to that point in time, but shows merely how many have it at that moment. For example, outbreaks of curable infectious diseases typically show initially a rapid rise in prevalence with time, followed by a decline as the disease runs its natural course through the population and diseased people recover. On the other hand, outbreaks of fatal infectious diseases may show low prevalence at any point in time even if many people have gotten the disease and died. Because prevalence indicates neither when the persons initially got the disease nor which previously diseased persons subsequently recovered or died, it is not a good measure of the overall rate, or risk, of getting the disease (see Ｉncidence).