The cross symbolizes the index case, filled circles infected persons, and hollow circles non-infected persons. The two large circles indicate increasingly less close contacts from center to periphery. In the most inner circle three out of ten close contacts were infected, a risk of 30%. Among intermediate contacts three out of 20, a risk of 15%. Among casual contacts the denominator is not known, but the risk is small, but nevertheless four persons were infected.

Thus, the risk is highest among those with the closest (longest) exposure, yet the total number of people infected by the index case outside those in close contacts may exceed the number infected among those in close contact.

This explains why, in a tuberculin skin test survey, a large proportion among those found infected have no recollection of ever having been exposed to a tuberculosis patient.

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