Editorial Infectivity of tuberculosis

The rapid and encouraging advances in chemo-therapy have led to a very rapid diminution in the duration of infectivity of patients suffering from infectious tuberculosis, and have made a significant contribution to the control of spread of the disease. There is thus a danger that other control measures, particularly contact tracing and examination, may be neglected because they may be considered to be time-consuming and to have a low yield. The potential infectiousness of the tuberculous patient was first suspected by Aristotle and these suspicions were echoed by Marten ' in the eight-eenth century who regarded close contact as important. In a series of elegant experiments Riley et al2 established beyond doubt the infec-tious potential of circulating air in a tuberculosis ward. It has been shown that, in smear-positive patients, droplet nuclei less than 10 gm in diameter and containing usually only one bacillus may transmit the disease. Thin watery sputum is more easily dispersed into infective particles than viscous material, and forced ventilatory manoeuvres such as coughing and singing are more efficient dispersing mechanisms than quiet breathing.4 5 The level of ventilation and re-circulation of air may also be of importance. Houk et all described an outbreak of tuberculosis in a submarine in which over one-third of the ship's crew developed evidence of infection. They related the spread of infection not only to direct contact but the ventilation system in each crew area. Other important factors are the closeness of contact of a tuberculin negative patient with an infectious patient, previous protection with BCG or chemoprophylaxis,7 and chemotherapy which reduces bacillary numbers8 and cough frequency9 within a few weeks. It is 25 years since Shaw and Wynn-Williams'0 studied the infectivity of pulmonary tuberculosis in relation to sputum status in Great Britain. In view of the great changes which have occurred in the epidemiology of tuberculosis in recent years