This is a made-up example.  Each line represents a case of sputum smear-positive tuberculosis.  In year 3, there are 12 incident cases.  A point-prevalence on March 15 in year 3 shows 24 prevalent cases.  We can also determine the person-time of infectiousness in year 3, by adding up all the months that each case contributes in this year.  In this example, this adds up to 288 months.

As discussed earlier, incident cases are necessary for the population to become exposed, but duration of infectiousness is an important modifier of exposure risk and thus also for risk of infection in the community.

The situation depicted here has been made up to simulate the situation underlying the assumptions discussed in the previous slide and may have been prevailing in the pre-chemotherapy era in certain settings.

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