It has been reported that a substantial proportion of the new cases arise from the previously infected population. Hence, it appears that exogenous reinfection and/or endogenous reactivation play a major role in the development of post-primary disease. Though the risk of disease associated with exogenous reinfection and endogenous reactivation has not been computed in Indian conditions, the data collected during a longitudinal study by National TB Institute, Bangalore was analysed to estimate the above mentioned risk rates.

The risk of disease associated with exogenous reinfection was 6.55% per year compared with 0.21% yearly due to endogenous reactivation. To test the validity of the computed risk rates these were applied to the interval between the 3rd and 4th surveys. It was then estimated that 64 new cases should have been diagnosed in that survey interval as against 57 cases actually diagnosed. It was also estimated that 1.9% of the total population would be having recent infection, 1.3% would be previously infected with recent reinfection and 32.7% with previous infection but no recent infection leaving 64.1% who are not infected at all (uninfected). Among the new cases diagnosed, 28% would have progressive primary disease, 41% cases arise due to exogenous reinfection and 31% due to endogenous reactivation. In other words, the 1.9% population with recent infection contributes 28% of the total new cases, the 1.3% reinfected population contributes 41% and the 32.7% previously infected population contributes the remaining 31% of the total new cases.