have been compiled to estimate the population size of PWID among the nation as a whole and for large metropolitan areas [7]–[10]. While these estimates are informative, they are based on past-year behavior, which is not the most relevant time period for calculating disease rates from national HIV surveillance data, which essentially measures lifetime behaviors [5], or for calculating rates from national hepatitis C survey data since ever use of injection drugs, even in the distant past, is a risk for HCV infection [11].
