CD4 and HIV infectious disease: For the back-calculation method, methods using CD4+ T-cell depletion models, methods using cohort studies and methods using serial prevalence data, HIV incidence is defined as the number of new HIV infections per unit time (year) or the instantaneous incident infections occurring at time t. However, for methods using cross-sectional prevalence data, HIV incidence is defined as the average hazard of new infections occurring during the interval.