Figure 1.

Mechanisms of emergence of drug-resistant tuberculosis (DRTB) in a community. Incidence of drug-sensitive TB (DSTB) contributes to the pool of prevalent DSTB (arrow a). Incident DRTB occurs either through acquired drug resistance (arrow b) or through transmitted/primary resistance (arrow c). For simplicity, this figure groups all drug-resistant strains together; in reality, the pool of resistant strains is heterogeneous and strains that are resistant to some drugs can acquire additional resistance to others.