The rapid development or growth of the disease in a community or region. Statistical thinking has made very much significant contributions to the understanding of such type of phenomena. A recent instance concerns the Acquired Immuno deficiency Syndrome (AIDS) where the complex statistical methods have been used to estimate the number of infected individuals, the incubation period of the disease, aetiology of the disease and monitoring and forecasting the course of the disease. The Figure 58, for instance, shows the annual numbers of new HIV infections in the US by risk group based on the deconvolution of AIDS incidence data.