Various factors are associated with an increased risk of becoming infected and subsequently developing disease (Table 1 and Fig. 1). Transmission is most efficient in poorly ventilated, crowded environments. Droplets become diluted once they enter the external environment and M. tuberculosis is rapidly destroyed by ultraviolet radiation. Following lung infection, multiplication and dissemination of the organism is contained once cell-mediated immunity develops at 2–12 weeks. The risk of an individual progressing to active disease in the months and first few years after infection depends on the bacterial load and the effectiveness of their immune defences. A depressed immune response at the time of infection increases the risk for progressive primary (including disseminated) disease. If.
