An unusual aggregation of the health events, real or perceived. The events might be grouped in the particular region or in some short period of time, or they might happen among the certain group of people, for instance, those having the particular occupation. The significance of studying such type of clusters as a means of determining the origins of the public health problems has long been recognized. In the year 1850, for instance, the Broad Street pump in London was identified as the main source of cholera by plotting cases on a map and noting the cluster around well. More recently, recognition of the clusters of relatively rare kinds of pneumonia and tumours among the young homosexual men led to the identi?cation of the acquired immune de?ciency syndrome (AIDS) and eventually to discovery of the human immune de?ciency virus (HIV).