It is obvious that there will be no exposure without incident infectious cases.  However, there are important modifiers which will increase or decrease the risk of exposure in the community at large.

The most important modifier is the duration of infectiousness of an incident infectious case of tuberculosis.  The longer the duration of infectiousness, the larger the number of susceptibles that come into contact with the case.  The shorter duration of infectiousness is, the fewer such encounters will take place.  This is important for considering intervention strategies: while it is difficult to influence incidence of tuberculosis directly, early case-finding and institution of appropriate curative chemotherapy can markedly reduce the duration of infectiousness.

Another factor that enters is the number of persons an infectious patient meets per unit of time of infectiousness.  This will greatly vary, for instance by:

Age: The extent of social interactions varies at different ages.

Gender: Men and women behave differently both in number of contacts and in the type of persons contacted.  Such differences may be more or less pronounced in different cultural contexts, depending the roles in society accorded to men and women.

Geographic location:  The most conspicuous differences are found between urban and rural areas.  The number of people who can become exposed by a single case of tuberculosis will be vastly larger in urban compared to rural settings.

Climatic conditions: Climates that allow much outdoor activity will lead to less relevant exposure than cold and un-hospital climates that force people to congregate indoors much of their time.

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