The reversal in the decline of tuberculosis during the past few years has been widely publicised. The World Health Organisation has declared a global emergency in the light of the rising incidence of the disease in developing countries, where the interaction between tuberculosis and AIDS can be devastating. In Zambia, for example, two thirds of patients who are diagnosed as having tuberculosis are HIV positive.1 But the problem is not confined to developing countries. In some American cities rates have risen alarmingly and strains resistant to multiple antibiotics have emerged.2 Notifications of tuberculosis have also risen in Britain since 1987, with drug resistant isolates increasing from 8% in 1987 to 14% in 1991.3 How should we respond to this re-emerging threat? The answer will differ among countries, but an understanding of the factors affecting transmission of the pathogen is crucial.

The recent development of molecular typing for Mycobacterium tuberculosis allows us to track the spread of individual …