All contacts of a smear positive individual are traced and tested according to the ring principle:

Ring 1 = Most intimate contacts: family and friends; statistical infection risk is 20%;

Ring 2 = Moderate contacts: ca. once a week; statistical infection risk is 4%;

Ring 3 = Occasional contacts: < 1/week; statistical infection risk is 0,3%.

A smear positive person infects 12-13 contacts. The extent of positive case finding in an inner ring influences contact testing in an outer ring (s).

Infection of contacts occurs mainly in the period preceding diagnosis-making. Once the diagnosis is known, effective hygiene is the rule (mandatory) till sputum conversion occurs, so as to limit further transmission.