Pathogenicity in Mycobacterium tuberculosis may be thought of as a multifactorial process with both pathogen and host-response effector molecules contributing to the process of infection, leading either to immunopathology and disease or control of infection and long-term persistence. Little is known about this at a genetic level, but it is becoming recognized that bacterial virulence constitutes the correct temporal and spatial regulation of many genes that may be necessary for a particular phase in infection in response to specific environmental cues.