Natural progression of events and outcome in an immunocompetent individual following exposure of human subjects (contacts of TB patients) to droplet nuclei containing
expectorated by a source case of sputum smear-positive pulmonary TB. Every year, ~50 million people worldwide are infected with M. tuberculosis. Complete elimination of tubercle bacilli is achieved in ~10% individuals only while in ~90% of infected individuals, bacterial growth is stopped but some bacilli survive and persist leading to latent M. tuberculosis infection (LTBI). The waning of dormant bacilli in persons with LTBI can be accelerated by therapy with isoniazid for 9 months (denoted by *). The vaccines currently in clinical trials are designed to prevent or delay the reactivation of latent infection in persons with LTBI (denoted by **).