Table 11– Classification of contacts around the index case according to the degree of exposure
1st circle of contacts (inner circle)Description
 Close household contactsThose who live in the same household as the infectious case. Household contacts are considered, by definition, to share breathing space on a daily basis with the source case.
 Close nonhousehold contactsClose nonhousehold contacts may include those persons with short exposure times to direct face-to-face streams of air with a particularly high density of infectious droplet nuclei, such as may occur during bronchoscopy or otorhinolaryngeal examination of patients with hence untreated sputum smear-positive tuberculosis, and similar situations. For all other close nonhousehold contacts, an arbitrarily defined cumulative exposure time of 8 h, if the index is sputum smear-positive, or 40 h, if only sputum culture-positive has been recommended as a guiding principle 18. This group also includes contacts with regular, prolonged contact with the source case, who share breathing space but do not live in the same household or who have spent time with the source case in a confined space, such as a car, sweatshop or prison cell. These may also include contacts, such as close friends and colleagues.
2nd circle of contacts (middle circle)
 Casual contactsThose who spent less time with the infectious case. These may include frequent visitors to the home, friends, relatives, school or class mates, colleagues at work or leisure contacts, members of a club or team, or passengers in adjoining seats during aircraft travel of >8 h 201.
3rd circle of contacts (outer circle)
 Community contactsThose living in the same community or attending the same school, sports club or workplace who may have had sporadic contact.
  • The information in this table is taken from 15, 16, 18, 22, 196–201.