Tuberculosis (TB) remains the most important infectious disease in the world. In Taiwan, the incidence of TB increased in recent years. The failure of control implies the necessity to reevaluate the epidemiology of Mycobacterium tuberculosis. It is widely thought that most cases of TB are caused by reactivation of a latent infection. Treatment programs have therefore focused on cure rates rather than tracking of additional cases. But recent studies showed that exogenous reinfection plays an important role in the development of TB. In addition, it seems that the higher the local incidence, the more important exogenous reinfection is. The question of exogenous reinfection versus endogenous reactivation has an impact on the distribution of resources for the prevention and treatment of TB. Based on these evidences, we hypothesize that exogenous reinfection is very important in the Taiwan endemic. Therefore, we design a series of studies to evaluate the individual contribution of exogenous reinfection and endogenous reactivation in the Taiwan endemic, and to realize the impact of exogenous reinfection. First, we will identify the patients with TB relapse after complete treatment. The M. tuberculosis isolates responsible for their initial and recurrent episodes will then be genotyped to clarify the percentage of exogenous reinfection and endogenous reinfection. In addition, we will try to detect the dormant M. tuberculosis bacilli in the lung tissue from patients just finished their anti-TB treatment by using highly sensitive molecular biologic methods to show the possibility of reactivation after complete treatment is low. Finally, we will prospectively survey the health care workers in a medical center by using chest radiograph at the 3rd weeks, and tuberculin skin test at the 3rd and 11th weeks after they expose to smear-negative culture-positive cases of TB. For those with abnormal findings, surveillance sputum mycobacterial studies will be performed. Once yielding M. tuberculosis, the isolates will be genotyped and compare with the DNA fingerprints of the index cases to demonstrate the transmission of TB.