Figure 1. Summary of the main assumptions in the model relating to the risks of developing disease. 1) General relationship between the risk of developing the first primary episode (during the first year after infection) and age at infection. An identical relationship is assumed to hold between the risk of exogenous disease and the age at reinfection and between the risk for endogenous disease and the current age of persons. 2) Risk of developing the first primary episode (or exogenous disease) in each year following initial infection (or reinfection), relative to that experienced in the first year after infection. The relationship was derived by using data on the interval between tuberculin conversion and disease among persons who were tuberculin negative at intake during the U.K. The Medical Research Council BCG trial during the 1950s (13). 3) Proportion of respiratory disease incidence manifested as sputum-positive (i.e., infectious). (Pers. comm. K. Styblo, Tuberculosis Surveillance Research Unit, and K. Bjartveit, Norwegian National Health Screening Service).