It has been stated that 1% annual risk of infection corresponds to an incidence of 100 sputum smear-positive cases per 100,000 population.

There are two major underlying assumptions necessary for this to be the case.  First, it is assumed that each case infects on average 10 persons per year.  Second, each case is on average infectious for two years.  If this is the case, then 100 incident sputum smear-positive cases will accomplish 2,000 transmissions in the population, that is they infect 2% of the population which is the risk of infection in the community.

That this is not always so, and actually probably very rarely so, is shown in the next two slides.

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